Add AUTHORS.rst file.

AUTHORS.rst

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+The PRIMARY AUTHORS are:
+
+- Andrey Antukh <niwi@niwi.be>
+- Jesus Espino Garcia <jespinog@gmail.com>
+- David Barragán Merino <dbarragan@dbarragan.com>
+- Alejandro Alonso <alejandro.alonso@kaleidos.net>
+
+Special thanks to Kaleidos Open Source S.L for provice time for greenmine
+development.
+
+And here is an inevitably incomplete list of MUCH-APPRECIATED CONTRIBUTORS --
+people who have submitted patches, reported bugs, added translations, helped
+answer newbie questions, and generally made greenmine that much better:
+
+- ...