[jfx11u] Integrated: 8273732: Clarify review policies for clean backports in JavaFX update releases

[Kevin Rushforth]

Clean backport of update to `CONTRIBUTING.md` to clarify that clean backports of approved fixes can be integrated without further review.

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Commit messages:
 - 8273732: Clarify review policies for clean backports in JavaFX update releases

Changes: https://git.openjdk.java.net/jfx11u/pull/53/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=53&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273732
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/53.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/53/head:pull/53

PR: https://git.openjdk.java.net/jfx11u/pull/53