RFR: Added build section [v2]

[Jesper Wilhelmsson]

On Mon, 3 May 2021 13:13:17 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:
> 
>   Fixed review comments

Thank you David! New version should address all your comments.

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