Automatic release notes bot

[Aaron Meurer]

As many of you know, our release notes for our releases are often not
very complete. They also tend to be the largest blocker whenever we
want to do a release.

For this last release (1.2), I spent about a month compiling the over
600 pull requests into the release notes
(https://github.com/sympy/sympy/wiki/Release-Notes-for-1.2). This
clearly isn't something that I'd like to do again.

Thus, I've been working on a way to get people to write their own
release notes entries. The idea is for everyone to write the entry in
the pull request description. A bot will then read the description
automatically and check if the notes are there and formatted
correctly. The pull request cannot be merged if there are no release
notes (you can write "NO ENTRY" to indicate the PR shouldn't have a
release notes entry). When the pull request is merged, the bot will
automatically add the entry to the wiki.

The idea is that since it is in the PR description, anyone can edit
it, both the PR author and the reviewers. The same goes with the
release notes being on the wiki.

I have an implementation of this idea at
https://github.com/sympy/sympy/pull/14942. I would like feedback from
the community on the wording of various things, before we go live with
it. I would also appreciate testing, as the bot is still new and
likely has some bugs.

I have also written an accompanying guide on how to write release
notes at https://github.com/sympy/sympy/wiki/Writing-Release-Notes. I
would appreciate any suggestions for this as well (feel free to edit
it on the wiki).

Aaron Meurer