RFR: 8231601: Update CONTRIBUTING.md to clarify process for contributing features plus Skara changes [v4]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'master' into 8231601-contributing
 - Address review coments
 - Fixed URL to WEBKIT-MEDIA-STUBS.md
 - Address review comments
 - Address review comments + additional fixes
 - 8231601: Update CONTRIBUTING.md to clarify process for contributing features plus Skara changes

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/303/files
  - new: https://git.openjdk.java.net/jfx/pull/303/files/9ee77cf7..1c41e9c6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=303&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=303&range=02-03

  Stats: 762084 lines in 11231 files changed: 387748 ins; 267778 del; 106558 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