document how to run the test suite

CONTRIBUTING.md

@@ -1,3 +1,16 @@
 ## Contributing
 
 I like when people send pull requests. It validates my existence. If you want to help out, check the [issue list](https://github.com/ggreer/the_silver_searcher/issues?sort=updated&state=open) or search the codebase for `TODO`. Don't worry if you lack experience writing C. If I think a pull request isn't ready to be merged, I'll give feedback in comments. Once everything looks good, I'll comment on your pull request with a cool animated gif and hit the merge button.
+
+### Running the test suite
+
+If you contribute, you might want to run the test suite before and after writing
+some code, just to make sure you did not break anything. Adding tests along with
+your code is nice to have, because it makes regressions less likely to happen.
+Also, if you think you have found a bug, contributing a failing test case is a
+good way of making your point and adding value at the same time.
+
+The test suite uses [Cram](https://bitheap.org/cram/). You'll need to build ag
+first, and then you can run the suite from the root of the repository :
+
+    make test