Draft a CONTRIBUTING file as suggested by Jody Garnett

See https://lists.osgeo.org/pipermail/discuss/2015-October/015197.html

git-svn-id: http://svn.osgeo.org/postgis/trunk@14290 b70326c6-7e19-0410-871a-916f4a2858ee

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
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+PostGIS development planning happens via a [Trac instance]
+(https://trac.osgeo.org/postgis/).
+
+In order for proper scheduling of contributions acceptance/merging it
+is recommended to file a ticket there and add your patch or a link
+to a patch or to a public git branch where your development is ongoing.
+
+Filing a ticket requires setting up an OSGeo account, which lets you
+contribute to every project of the [Open Source Geospatial Foundation]
+(http://www.osgeo.org/).
+
+As we understand you might not want to setup a new account, simply
+filing a pull request on one of the [PostGIS code mirrors]
+(https://trac.osgeo.org/postgis/wiki/CodeMirrors) is also welcome,
+but it is easier for such a contributions to miss our radars unless
+it _also_ has a matching Trac ticket.
+
+If you intend to be involved for more than an occasional patch we
+recommend you to subscribe to the [development mailing list]
+(https://lists.osgeo.org/mailman/listinfo/postgis-devel).