Add a paragraph describing the automated tests

CONTRIBUTING.rst

@@ -39,6 +39,42 @@ It is also recommended to add some unit or end-to-end tests for the proposed
 changes.
 
 
+Tests
+-----
+
+There are two test suites for biboumi:
+
+- unit tests that can be run simply using `make check`.
+  These tests use the Catch test framework, are written in pure C++
+  and they should always succeed, in all possible build configuration.
+
+- a more complex end-to-end test suite. This test suite is written in python3,
+  uses a specific IRC server (`charybdis`_), and only tests the most complete
+  biboumi configuration (when all dependencies are used). To run it, you need
+  to install various dependencies: refer to fedora’s `Dockerfile.base`_ and
+  `Dockerfile`_ to see how to install charybdis, slixmpp, botan, litesql, an
+  ssl certificate, etc.
+
+  Once all the dependencies are correctly installed, the tests are run with
+
+  `make e2e`
+
+  To run one or more specific tests, you can do something like this:
+
+  `make biboumi && python3 ../tests/end_to_end  self_ping  basic_handshake_success`
+
+  This will run two tests, self_ping and basic_handshake_success.
+
+  To write additional tests, you need to add a Scenario
+  into `the __main__.py file`_. If you have problem running this end-to-end
+  test suite, or if you struggle with this weird code (that would be
+  completely normal…), don’t hesitate to ask for help.
+
+
+All these tests automatically run with various configurations, on various
+platforms, using gitlab CI.
+
+
 Coding style
 ------------
 Please try to follow the existing style:
@@ -55,3 +91,7 @@ Please try to follow the existing style:
 .. _gitlab merge request: https://lab.louiz.org/louiz/biboumi/merge_requests/new
 .. _github pull request: https://github.com/louiz/biboumi/pulls
 .. _XMPP chatroom: xmpp:biboumi@muc.poez.io
+.. _Dockerfile.base: docker/biboumi-test/fedora/Dockerfile.base
+.. _Dockerfile: docker/biboumi-test/fedora/Dockerfile
+.. _charybdis: https://github.com/charybdis-ircd/charybdis
+.. _the __main__.py file: tests/end_to_end/__main__.py