Add test guidelines to CONTRIBUTING.md
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index f55fc28..83e04f8 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -68,6 +68,8 @@
Mbed TLS includes a comprehensive set of test suites in the `tests/` directory that are dynamically generated to produce the actual test source files (e.g. `test_suite_mpi.c`). These files are generated from a `function file` (e.g. `suites/test_suite_mpi.function`) and a `data file` (e.g. `suites/test_suite_mpi.data`). The function file contains the test functions. The data file contains the test cases, specified as parameters that will be passed to the test function.
+[A knowledgebase article describing how to add additional tests is available on the Mbed TLS website](https://tls.mbed.org/kb/development/test_suites).
+
A test script `tests/scripts/basic-build-test.sh` is available to show test
coverage of the library. New code contributions should provide a similar level
of code coverage to that which already exists for the library.