RFR: Added build section [v3]

[David Holmes]

On Mon, 3 May 2021 13:20:23 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

>> Instructions on how to build the JDK.
>
> Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Link to build instructions

LGTM! Thanks Jesper.

David

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Marked as reviewed by dholmes (Author).

PR: https://git.openjdk.java.net/guide/pull/56