RFR: Added build section

[Jesper Wilhelmsson]

Instructions on how to build the JDK.

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Commit messages:
 - Added build section

Changes: https://git.openjdk.java.net/guide/pull/56/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=56&range=00
  Stats: 71 lines in 1 file changed: 71 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/guide/pull/56.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/56/head:pull/56

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