Fix #324: Add 'Running Tests' section

docs/CONTRIBUTING.rst

@@ -1,6 +1,54 @@
+Running Tests
+=============
+
+The proper way how to run unit tests is using `tox
+<http://tox.readthedocs.io/>`_. See `tox.ini` configuration file in the
+topmost directory of the project's repository.
+
+For most cases, you need to work against tox's `py27` environment. This
+will just invoke `pytest` command. For full set of tests, omit the `-e
+py27` argument. This will invoke code analysis, security analysis and
+code coverage checks. And any other environments which will appear in the
+`tox.ini` file in the future.
+
+Examples
+--------
+
+Purge environment (this will remove all untracked files/dirs and
+uncommited changes!) and run tests::
+
+    $ git clean -fdxe '/.vagrant'
+    $ tox -e py27
+
+Just recreate tox's `py27` environment and run the tests::
+
+    $ tox -e py27 -r
+
+Just rerun tests in the existing `py27` enviromnent::
+
+    $ tox -e py27
+
+Pass additional arg(s) to `pytest`, e.g. `-x` to exit on first
+error/failed test::
+
+    $ tox -e py27 -- -x
+
+Run only tests in the given file or directory::
+
+    $ tox -e py27 -- path/to/test_file.py/or/test_dir
+
+For more `tox` or `pytest` options, see the corresponding help/man pages.
+
+*Note: Consider using detox which makes efficient use of multiple CPUs
+by running all possible activities in parallel. It has the same options
+and configuration that tox has.*
+
 Development
 ===========
 
+In most cases, you don't need to deploy your development instance. Please,
+refer to `Running Tests` section first.
+
 We have two mechanisms for quickly setting up a development environment,
 `docker-compose` and `vagrant`.
 
@@ -186,7 +234,7 @@ Then uncomment the fedmsg signing configuration in
 `fedmsg.d/module_build_service.py`.
 
 Historical Names of Module Build Service
-----------------------------------------
+========================================
 
 - Rida
 - The Orchestrator