RFR: Add gitignore for build directory

[Jesper Wilhelmsson]

On Wed, 27 May 2020 14:30:39 GMT, Martijn Verburg <github.com+180840+karianna 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.
>
>> 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?

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 ��

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