RFR: Add gitignore for build directory

[Martijn Verburg]

On Wed, 27 May 2020 14:36:11 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

> Please follow the instruction above to open an issue in JBS to get your user name tied to your GitHub account. Robin is
> standing by to approve it ��

Hi Jesper, sadly I'm only a Contributor, not an Author, and therefore cannot open an issue in JBS...

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PR: https://git.openjdk.java.net/guide/pull/12