[jfx20u] RFR: 8304008: Update README.md and CONTRIBUTING.md for jfx update repos

[Johan Vos]

On Fri, 10 Mar 2023 23:02:18 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> Update README and CONTRIBUTING guidelines for this repo as appropriate for update releases (basically a copy from jfx17u with 17 changed to 20).

Marked as reviewed by jvos (Reviewer).

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

PR: https://git.openjdk.org/jfx20u/pull/2