Add proposals.md document which explains the proposal process (#11006)

The cabal developers are interested to adopt a proposal process, the
details are found in the `proposals.md` file.

The process is designed to make developers feel empowered to make decisions.

* It is light-weight, a PR is opened and discussed on a repo with a fixed discussion period.
* It is developer-led, final decisions are made by developers at the Cabal developers meeting.
* It is flexible, there is no formal voting procedure, decisions are made by rough consensus.

Overall, we hope that this will allow developers to move the cabal project forward.