Make it more obvious how tests can be run locally

Byron · Byron · commit 09d2ae5f1ca4 · 2017-12-11T16:57:47.000+01:00
See gitpython-developers#674 for reference. [skip CI]
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
@@ -1,6 +1,7 @@
 ### How to contribute
 
 * [fork this project](https://github.com/gitpython-developers/GitPython/fork) on github
+* For setting up the environment to run the self tests, look at `.travis.yml`.
 * Add yourself to AUTHORS.md and write your patch. **Write a test that fails unless your patch is present.**
 * Initiate a pull request
 
