RFR: added a note on JDK version to use for Gradle build. [v2]

[Athijegannathan Sundararajan]

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

Athijegannathan Sundararajan has updated the pull request incrementally with one additional commit since the last revision:

  link to Gradle compatibility matrix page

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Changes:
  - all: https://git.openjdk.org/jextract/pull/75/files
  - new: https://git.openjdk.org/jextract/pull/75/files/96982412..19f83545

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=75&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=75&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 1 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