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

[Jorn Vernee]

On Thu, 22 Sep 2022 11:14:35 GMT, Athijegannathan Sundararajan <sundar at openjdk.org> wrote:

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

I suggest adding a link to the Gradle compatibility matrix: https://docs.gradle.org/current/userguide/compatibility.html#java

-------------

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