Add entry for HF Slack in communications.md

We have officially settled on Slack as our informal, asynchronous and unofficial chat application. There are new points of difference decided on a recent call between myself, Michael, and Tom:

1. Anyone is allowed in. We'll have to determine a moderation policy as a result, but we feel an open forum is better in this case.

2. We will announce this officially, and provide an open invite link, which will be posted on social media and be given its own entry on the HF website.

3. The existing rules written in communications.md are already sound with respect these changes, and need only a small update with the relevant info.

communication.md

@@ -4,6 +4,7 @@ This document describes how the Haskell Foundation Board communicates
 among itself and to the broader Haskell community.
 
 In thinking about communication, it is helpful to identify (at least) four different scenarios:
+
 1. Board members communicating with one another
 2. Community members talking to the Board
 3. The Board communicating out to the community
@@ -67,7 +68,7 @@ best judgement.
    e. Certain sensitive topics (e.g. personnel, sponsor relationships, unratified potential partnerships, etc.)
    are communicated by direct mail or via a mailing list without public archives.
 
-1. Communication from the Board to the community (Scenario 3) and between members of the community (Scenario 4)
+2. Communication from the Board to the community (Scenario 3) and between members of the community (Scenario 4)
    go via the [Haskell Foundation category](https://discourse.haskell.org/c/haskell-foundation/11)
    of the [Haskell Discourse instance](https://discourse.haskell.org/).
 
@@ -79,9 +80,9 @@ best judgement.
 
    c. Members of the HF will be sure to monitor the traffic in this Category.
 
-1. All task forces, committees and ad hoc groups are invited to use
-   the Haskell Foundation chat service (details TBD), because it is great for
-   lightweight, private and informal conversations.
+3. All task forces, committees, ad hoc groups, and anyone else interested in the Haskell Foundation's activities are invited to use
+   the [Haskell Foundation Slack](https://haskell-foundation.slack.com/), 
+   because it is great for lightweight, private and informal conversations. 
 
    a. In recognition of the fact that not all Board members will be able to
    monitor chat at all times, anyone participating in a chat discussion is
@@ -112,7 +113,7 @@ best judgement.
    apply, chat is considered private and is not subject to our transparency
    policy. Chat conversations are not archived.
 
-1. Official communication between Board members goes through the mailing list. For example, an asynchronous
+4. Official communication between Board members goes through the mailing list. For example, an asynchronous
    vote is conducted only via the mailing list, never in other modes. Board members
    are *not* required to keep up with Discourse, and so if you want to reach the entire
    Board, email `board@haskell.foundation`.
@@ -121,7 +122,7 @@ best judgement.
 
    * To address the Board specifically (Scenario 2), email `board@haskell.foundation`.
    * To address the community (Scenario 4), including Board members, use Discourse.
-   
+
 ## Synchronous communication (meetings)
 
 The rules of official Board meetings (Scenario 1) are laid out [here](board.md).
@@ -145,14 +146,14 @@ The rules of official Board meetings (Scenario 1) are laid out [here](board.md).
    release (Scenario 3); and GitHub is good for repos that expect a good deal of external
    contribution.
 
-1. Both Google Drive and GitLab are intended to be used for Board-internal communication,
+2. Both Google Drive and GitLab are intended to be used for Board-internal communication,
    communication with and within the Executive Team, communication within Task Forces as well
    general communication of the Foundation. (Scenarios 1 and 3)
 
-1. The Board will establish access policies for different parts of its Google Drive and GitLab
+3. The Board will establish access policies for different parts of its Google Drive and GitLab
    which the Chair and the Secretary will implement in their role as adminstrators.
 
-1. The Executive Team establishes their own practices for data storage and document management
+4. The Executive Team establishes their own practices for data storage and document management
    and informs the Board about those.
 
 ### Current status

[Michael Snoyman]

As I'm writing up the announcement to the community, I realize that we need to offer some way to request an invitation. I think an admin, likely @emilypi, would need to do something at https://haskell-foundation.slack.com/admin/shared_invites.

[Emily Pillmore]

So there's no general way to do this with Slack, unfortunately. It's "anyone with the link is invited" - we can only shut off the need for admin approval, which it is. We can pipe people to email for making slack requests, like contact@haskell.foundation (or slack@hf if we want a dedicated email), and publicly post our Slack invites. The FP Slack has a static invite link on the Haskell reddit, and it seems to be accessible enough from that location to attract a few thousand members.

These are the only settings available:

[Richard Eisenberg]

The FP Slack has a static invite link on the Haskell reddit, and it seems to be accessible enough from that location to attract a few thousand members.

Does that mean that one email-generated link can be used thousands of times? That sounds like a general signup link to me.

If I'm misunderstanding this, though, then maybe a slack@hf address is a good idea.

[Emily Pillmore]

I believe it's a general signup link, yeah. But don't quote me on that. I'm not sure if it can be reused. I have only seen that it expires after 4 weeks, so it needs to be continuously updated.

[Michael Snoyman]

Let me turn this around: I'm asking the Slack admin to tell me "how do I advertise to people that they can easily get onto our Slack instance?" If it's a refreshed URL, an email address, "ask a friend," etc, all are acceptable. I just want to know the official way to tell people to do it.

[Richard Eisenberg]

@emilypi ping. We discussed this briefly earlier today, but I think this task is in your court before this MR can merge.

[Richard Eisenberg]

@emilypi ping again?

[Richard Eisenberg]

The link is https://join.slack.com/t/haskell-foundation/shared_invite/zt-mjh76fw0-CEjg2NbyVE8rVQDvR~0F4A. I went to add this myself, but I can't edit this MR, as it's o @emilypi's fork. @emilypi could you please add this link to the page so that we can merge? Thanks.