This adds in a useful file to know how to compiled this theme. As there was previously a contributing file with instruction on this in the GitHub repo that was not brought into svn, this fixes that.
Props kjeller, nielslange, sabernahardt.
Fixes#46108.
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git-svn-id: http://core.svn.wordpress.org/trunk@57900 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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