RFR: Added build section [v3]

Wed May 5 15:09:00 UTC 2021

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

Marked as reviewed by iris (Reviewer).

-------------

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