RFR: Update the readme to include IntelliJ setup instructions

[Robin Westberg]

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

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Commit messages:
 - Update readme to include IntelliJ setup

Changes: https://git.openjdk.java.net/skara/pull/998/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=998&range=00
  Stats: 12 lines in 2 files changed: 10 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