Integrated: Added build section

[Jesper Wilhelmsson]

On Wed, 28 Apr 2021 00:24:44 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

> Instructions on how to build the JDK.

This pull request has now been integrated.

Changeset: 19696bef
Author:    Jesper Wilhelmsson <jwilhelm at openjdk.org>
URL:       https://git.openjdk.java.net/guide/commit/19696bef5d3baa035be12d8ebe0a119bac4e0f1d
Stats:     72 lines in 1 file changed: 72 ins; 0 del; 0 mod

Added build section

Reviewed-by: dholmes, iris

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