DOC: Link to contributing guide (googleapis#28)

Full contributing (and testing) instructions are included in the docs.
http://pandas-gbq.readthedocs.io/en/latest/contributing.html

I didn't find these docs (GH27) because I didn't expect them to be in
the user documentation, and I'm used to looking for a CONTRIBUTING file.

By creating a CONTRIBUTING.md file, GitHub users will be more likely to
find them. See: https://github.com/blog/1184-contributing-guidelines

CONTRIBUTING.md

@@ -0,0 +1,5 @@
+# Contributing
+
+See the [contributing guide in the pandas-gbq
+docs](http://pandas-gbq.readthedocs.io/en/latest/contributing.html).
+