[jfx24u] RFR: 8304008: Update README.md and CONTRIBUTING.md for jfx update repos

[Kevin Rushforth]

This is a copy of the jfx23u README and CONTRIBUTING docs, changing 23 to 24.

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Commit messages:
 - 8304008: Update README.md and CONTRIBUTING.md for jfx update repos

Changes: https://git.openjdk.org/jfx24u/pull/2/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx24u&pr=2&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304008
  Stats: 269 lines in 2 files changed: 1 ins; 256 del; 12 mod
  Patch: https://git.openjdk.org/jfx24u/pull/2.diff
  Fetch: git fetch https://git.openjdk.org/jfx24u.git pull/2/head:pull/2

PR: https://git.openjdk.org/jfx24u/pull/2