Add new, small "Testing" section to CONTRIBUTING.rst

"Drive-through" contributor here. As in many other projects, one of the
most time-consuming step before starting to do any actual work is
finding in my bash history (!) the right tox/pytest/test_foo.sh
incantation. No surprise maintainers have to beg for test(s) all the
time ;-P

Signed-off-by: Chris Lamb <lamby@debian.org>

CONTRIBUTING.rst

@@ -17,6 +17,16 @@ changes and submit a *merge request* via Salsa.  See `Gitlab documentation
 
 You can also submit bugs about Debian specific issues to the Debian bug tracker. 
 
+Testing
+=======
+
+diffoscope's test suite relies on `pytest <https://docs.pytest.org/>`__;
+to run all tests use ``pytest[-3]``. For faster interactive development
+here's an example of how to run a (much) smaller subset of tests:
+
+    pytest  -v --exitfirst -k lib tests/comparators/test_elf.pyc [ --pdb ]
+
+More options in `pytest -h`.
 
 Add a comparator
 ================