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

Thu Sep 22 11:28:07 UTC 2022

Added missing note to README.md on JDK version required for Gradle version.

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

Changes: https://git.openjdk.org/jextract/pull/75/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=75&range=00
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/75.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/75/head:pull/75

PR: https://git.openjdk.org/jextract/pull/75