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

Wed Feb 3 09:13:43 UTC 2021

On Tue, 2 Feb 2021 16:00:36 GMT, Robin Westberg <rwestberg at openjdk.org> wrote:

>> 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

Looks good!

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Marked as reviewed by ehelin (Reviewer).

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