RFR: added a note on JDK version to use for Gradle build.

Fri Sep 23 11:11:05 UTC 2022

Same change from 'master' to 'panama' branch

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Commit messages:
 - added a note on JDK version to use for Gradle build.

Changes: https://git.openjdk.org/jextract/pull/77/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=77&range=00
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/77.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/77/head:pull/77

PR: https://git.openjdk.org/jextract/pull/77