RFR: Add gitignore for build directory

[Martijn Verburg]

On Wed, 27 May 2020 14:21:38 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

> Looks good. Thanks for adding this! I've been meaning to :-)
> I'll sponsor this as you are not an author in this project yet.

Thanks.  Any way of getting the bot to recognize that I _have_ signed the OCA already?

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