Update CONTRIBUTING.md to clarify process for contributing features plus Skara changes

The documented policies for contributing new features to JavaFX could be made more clear, specifically the need to discuss API changes on the mailing list before working on it. The following PR included a meta-discussion that prompted this desired process improvement:

https://github.com/javafxports/openjdk-jfx/pull/499

I wrote up some notes here that can be used as the basis for updating the policies:

https://github.com/kevinrushforth/openjdk-jfx/blob/notes/doc-files/kcr/new-features.md

Additionally, we need to make changes in the contributing guide to align with recent (and not-so-recent) changes in the Skara tooling. For example:

* The Skara bot cross-links the PR and JBS issue, so need not be done manually
* The Skara bot automatically resolves the JBS issue when the PR is integrated, so need not be done manually
* We can suggest the use of the "/reviewers 2" and "/csr" commands.