Add CONTRIBUTING file

CONTRIBUTING

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+The development tracker for cartodb-postgresql is on github:
+http://github.com/cartodb/cartodb-postgresql/
+
+Bug fixes are best reported as pull requests over there.
+Features are best discussed on the mailing list:
+https://groups.google.com/d/forum/cartodb
+
+Adding features to the extension
+--------------------------------
+
+Extension features are coded in scripts found under the
+"scripts-available" directory. A feature can be a single function
+or a group of function with a specific scope.
+
+The "scripts-enabled" directory contains symlinks to the scripts
+in "scripts-available". Any symlink in that directory is automatically
+included in the extension. Numbering can be used to enforce the order
+in which those scripts are loaded.
+
+Scripts would be best coded in a way to be usable both for creation
+and upgrade of the objects. This means using CREATE OR REPLACE for
+the functions, and whatever it takes to check existance of any previous
+version of objects in other cases.
+
+When used as an extension (probably always from version 0.2.0 onwards)
+all the objects will be installed in a "cartodb" schema. Take this into
+account to fully-qualify internal calls to avoid (possibly dangerous)
+name clashes.
+
+Every new feature (as well as bugfixes) should come with a testcase,
+see next session.
+
+Writing testcases
+-----------------
+
+Tests reside in the test/ directory.
+You can find information about how to write tests in test/README