An improved README experience for PostgreSQL

Hello Hackers. We’re proposing an improved README for PostgreSQL that
includes more helpful links for prospective PostgreSQL contributors and has
a nicer presentation.

Although development does not take place on GitHub or GitLab for
PostgreSQL, many developers might view the PostgreSQL source code using one
of those mirrors (I do myself). Since both support Markdown files, a
Markdown version of the README (as README.md) gets presentational benefits
that I think are helpful.

For a head-to-head comparison of what that looks like, review the current
README and a proposed README.md version below:

Current version:

https://github.com/andyatkinson/postgres/blob/master/README

Markdown README.md version on GitHub:

https://github.com/andyatkinson/postgres/blob/e88138765750b6f7898089b4016641eee01bf616/README.md

---- Feedback Requested ----

Samay Sharma are both interested in the initial developer experience for
PostgreSQL. We had a chat about the role the README plays in that, while
it's a small role, we thought this might be a place to start.

We'd love some feedback.

Prospective contributors need to know about compilation, the mailing lists,
and how the commitfest events work. This information is scattered around on
wiki pages, but we're wondering if more could be brought into the README
and whether that would help?

If you do check out the new file, we'd love to know whether you think
there's useful additions, or there's content that's missing.

If there's any kind of feedback or consensus on this thread, I'm happy to
create and send a patch.

Thanks for taking a look!

Andrew Atkinson w/ reviews from Samay Sharma