Adds docs for building and running unit tests to CONTRIBUTING.md

Author: Alomir

CONTRIBUTING.md

@@ -15,3 +15,14 @@ make clean
 # list all make commands
 make help
 ```
+## Testing
+
+SIPNET also uses `make` to build and run its unit tests. This can be done with the following commands:
+```shell
+# Compile tests
+make test
+# Run tests
+make testrun
+# Clean after tests are run
+make testclean
+```