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

[Robin Westberg]

On Tue, 2 Feb 2021 13:53:06 GMT, Erik Helin <ehelin at openjdk.org> wrote:

>> 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 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 ��

Sure, but the `Building` section still says that 14 or later is needed, and I guess that is still actually true. :)

> 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 ..."?

Makes sense, I'll reword it.

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