change readme to have links to PostGIS contributing instructions and manuals

git-svn-id: http://svn.osgeo.org/postgis/trunk@14292 b70326c6-7e19-0410-871a-916f4a2858ee

Author: robe2

README.md

@@ -7,4 +7,8 @@ Actual readme is [here](README.postgis).
 Official code repository, issue tracker and wiki:
 https://trac.osgeo.org/postgis/
 
+If you would like to contribute to this project, please refer to our
+[contributing guidelines](CONTRIBUTING.md).
+
 Project homepage: http://postgis.net/
+PostGIS Manuals: http://postgis.net/documentation