DOC: link to travis setup instructions in CONTRIBUTING.md

Author: y-p

CONTRIBUTING.md

@@ -18,7 +18,7 @@ your contribution or address the issue you're having.
 
 - When submitting a Pull Request
   - **Make sure the test suite passes**., and that means on python3 as well.
-    You can use "test_fast.sh", or tox locally and/or enable Travis-CI on your fork.
+    You can use "test_fast.sh", or tox locally and/or [enable Travis-CI](http://about.travis-ci.org/docs/user/getting-started/) on your fork.
   - We suggest you enable Travis-CI on your fork, to make it easier for the team
      to see that the PR does indeed pass all the tests.
   - Back-compatiblitiy **really** matters. Pandas already has a large user-base and