[jdk21u-dev] RFR: 8343474: [updates] Customize README.md to specifics of update project.

[Goetz Lindenmaier]

Customize the README.md to jdk21u.

Plan is to backport this to 17 and maybe older jdks and adapt the paths etc correspondingly.

Should we put there more information?

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Commit messages:
 - 8343474: [updates] Customize README.md to specifics of update project

Changes: https://git.openjdk.org/jdk21u-dev/pull/1118/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1118&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343474
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1118.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1118/head:pull/1118

PR: https://git.openjdk.org/jdk21u-dev/pull/1118