RFR: Update the readme to include IntelliJ setup instructions [v3]

[Robin Westberg]

> Adds a section on how to build and test Skara using IntelliJ IDEA.

Robin Westberg has updated the pull request incrementally with one additional commit since the last revision:

  Clarify JDK location

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/998/files
  - new: https://git.openjdk.java.net/skara/pull/998/files/58f33fd7..b75f0a86

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=998&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=998&range=01-02

  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/998.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/998/head:pull/998

PR: https://git.openjdk.java.net/skara/pull/998