git: openjdk/jfx21u: master: 8304008: Update README.md and CONTRIBUTING.md for jfx update repos

[Kevin Rushforth]

Changeset: d689dc12
Author:    Kevin Rushforth <kcr at openjdk.org>
Date:      2023-07-19 22:08:35 +0000
URL:       https://git.openjdk.org/jfx21u/commit/d689dc12751660c14e558c37e82f8dc71d9a8037

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

Reviewed-by: jvos
Backport-of: 9733e7a925df2cf47d5994b55dbb563d7fa90e94

! CONTRIBUTING.md
! README.md