Introduce CONTRIBUTING.md (#653)

I used a copy of Citus' CONTRIBUTING.md as a starting point.

I removed sections that are not relevant to pgcopydb, and added some
more sections about documentation and running tests.

I also added references to the CONTRIBUTING.md file in the README.md and
in documentation page about installing pgcopydb. While I was at it, I
also removed the outdated installation instructions from the README.md
file, and replaced them with a reference to the documentation page.

Co-authored-by: Dimitri Fontaine <dim@tapoueh.org>