CONTRIBUTING.rst: Updated to mention new `coverage` tox environment.

Also point out that it is possible to run test environments in
parallel using the `detox` tool.

CONTRIBUTING.rst

@@ -1498,6 +1498,10 @@ option when running tox::
 
   tox -e py37
 
+If you do have multiple python versions installed and would like to test against
+multiple versions, then we recommend using `detox <https://github.com/tox-dev/detox>`_,
+just run it with the same arguments you would give `tox`.
+
 Linting is performed separately from testing. In order to run the linting step which
 consists of running the ``pycodestyle`` and ``pylint`` tools, run the following::
 
@@ -1574,6 +1578,19 @@ can run ``tox`` with ``-r`` or  ``--recreate`` option.
      ./setup.py test --addopts 'tests/frontend/buildtrack.py::test_build_track'
 
 
+Observing coverage
+~~~~~~~~~~~~~~~~~~
+Once you have run the tests using `tox` (or `detox`), some coverage reports will
+have been left behind.
+
+To view the coverage report of the last test run, simply run::
+
+  tox -e coverage
+
+This will collate any reports from separate python environments that may be
+under test before displaying the combined coverage.
+
+
 Adding tests
 ~~~~~~~~~~~~
 Tests are found in the tests subdirectory, inside of which