Fixes #106

Since script is served separately from the main source code (can be opened over
https://get-ghcup.haskell.org) - mention the script license to the reciever.

Provide some basic instructive information to someone who views the script.

Fixes #28

Previously we would prefer .bash_profile
if no .bashrc exists, but that seems a bit
far-fetched.

.bash_profile is for login shells, .bashrc
is sourced for interactive non-login shells,
but most .bash_profiles also explicitly source
.bashrc, so it's hard to go wrong with it.