RFR: 8231601: Update CONTRIBUTING.md to clarify process for contributing features plus Skara changes [v2]

[Kevin Rushforth]

> This PR updates the `CONTRIBUTING.md` guide to address the following:
> 
> 1. Clarify the process for adding new features / API changes, specifically that they must be discussed on the mailing
> list as the first step. 2. Add a link to the mailing list in the section regarding contributing bug fixes.
> 3. Remove the text about cross-linking the PR and JBS issue, and add a note that the Skara tooling takes care of this
> 4. Remove the section about manually resolving the JBS issue, and add a note that the Skara bot automatically does this
> when the PR is integrated. 5. Suggest the use of the "/reviewers 2" and "/csr" commands when appropriate
> 6. Update the note regarding which JDK(s) to use.

Kevin Rushforth has updated the pull request incrementally with one additional commit since the last revision:

  Fixed URL to WEBKIT-MEDIA-STUBS.md

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/303/files
  - new: https://git.openjdk.java.net/jfx/pull/303/files/9840ffea..becd269f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=303&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=303&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/303.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/303/head:pull/303

PR: https://git.openjdk.java.net/jfx/pull/303