RFR: Update the readme to include IntelliJ setup instructions [v2]

[Erik Helin]

On Tue, 2 Feb 2021 13:44:35 GMT, Robin Westberg <rwestberg at openjdk.org> wrote:

>> Adds a section on how to build and test Skara using IntelliJ IDEA.
>
> Robin Westberg has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:
> 
>   Update readme to include IntelliJ setup

README.md line 229:

> 227: the terminal, which will download a suitable JDK. Configure IntelliJ to use it
> 228: at `File → Project Structure → Platform Settings → SDKs → + → Add
> 229: JDK... → <skara-folder>/.jdk/openjdk-15.0.1_osx-x64_bin/jdk-15.0.1.jdk/Contents/Home`.

Maybe generalize this a bit for Windows and/or Linux users? Or just state something like "On macOS this would be configured like the following ..."?

README.md line 226:

> 224: IDE when working on Skara you can simply open the root folder and the project
> 225: should be automatically imported. You will need to configure a Platform SDK that
> 226: is JDK 14 or above. Either set this up manually, or [build](#building) once from

We are using JDK 15 now ��

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