Update the contribution guide to describe the test suite.

CONTRIBUTING.md

 The SWIFT source code is using a variation of the 'Google' formatting style. 
 The script 'format.sh' in the root directory applies the clang-format-3.8
 tool with our style choices to all the SWIFT C source file. Please apply 
-the formatting script to the files before submitting a merge request.
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+the formatting script to the files before submitting a merge request.
+
+The SWIFT code comes with a series of unit tests that are run automatically 
+when a push to the master branch occurs. The suite can be run by doing a `make 
+check` in the root directory. Please check that the test suite still
+runs with your changes applied before submitting a merge request and add 
+relevant unit tests probing the correctness of new modules. An example of how
+to add a test to the suite can be found by considering the tests/testGreeting 
+case.
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