RFR: 7902912: Update IntelliJ Plugin build Readme

[Thomas Stuefe]

On Fri, 30 Apr 2021 08:58:00 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> Hi, may I have reviews for this small documentation fix please? I am not a CODETOOLS committer so I may need someone to sponsor this fix.
> 
> Since I just built the intellij plugin in 2020.3, I stumbled about a couple of ambiguous infos which are not immediately clear to outsiders.
> 
> -> some UI features changed in Intellij
> -> Clarified the meaning of JTREG_HOME
> -> One thing I was not sure about was the required SDK level to build the plugin. The documentation says "Any JDK 8 is fine" which I took as "JDK8 is required" - proceeded to download JDK 8 but then I was unable to choose this JDK as a SDK for the IDEA JDK. It was, however, happy with a JDK 15 I gave it, and proceeded to build the plugin without any issues. Therefore I changed the sentence to "Any JDK 8++ is a fine choice".

Thanks, Maurizio! I took your suggestion. Feel free to sponsor this change, as I said, I am not a committer for codetools.

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PR: https://git.openjdk.java.net/jtreg/pull/8