RFR: 7902912: Update IntelliJ Plugin build Readme

[Thomas Stuefe]

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".

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Commit messages:
 - start

Changes: https://git.openjdk.java.net/jtreg/pull/8/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=8&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-7902912
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/8.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/8/head:pull/8

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