RFR: 8312882: Update the CONTRIBUTING.md with pointers to lifecycle of a PR

[Jesse Glick]

@jaikiran in https://github.com/openjdk/jdk/pull/12871#issuecomment-1612310113 pointed me to a resource I had not previously found. https://openjdk.org/contribute is fine at a high level but still says things like

> When your change is ready, send a message to the appropriate development list…

which clearly predates Skara, and does not apparently link to anything more current. Whereas this https://github.com/openjdk/jdk/blob/master/doc/building.md is thorough and discoverable, finding accurate guidelines for proposing pull requests was tough.

-------------

Commit messages:
 - 8312882: Update the CONTRIBUTING.md with pointers to lifecycle of a PR

Changes: https://git.openjdk.org/jdk/pull/14716/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14716&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312882
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14716.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14716/head:pull/14716

PR: https://git.openjdk.org/jdk/pull/14716