RFR: Section on cloning [v8]

[Jesper Wilhelmsson]

On Fri, 20 Aug 2021 00:26:55 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

>> Added a section on cloning from GitHub, and an example of building on Mac. Also fixed two minor bugs that caused problems with syntax highlighting in Atom.
>
> Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Introduced SSH earlier

I tried with a newly created GitHub account and verified that the SSH key is required already when cloning. So I added an initial clone example that uses https. Also added references to the Skara documentation.

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PR: https://git.openjdk.java.net/guide/pull/59