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

[Kevin Rushforth]

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

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

Changes: https://git.openjdk.org/jfx20u/pull/2/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx20u&pr=2&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304008
  Stats: 267 lines in 2 files changed: 1 ins; 254 del; 12 mod
  Patch: https://git.openjdk.org/jfx20u/pull/2.diff
  Fetch: git fetch https://git.openjdk.org/jfx20u pull/2/head:pull/2

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