docs: Add a section explaining how to write functional tests

CONTRIBUTING.md

@@ -191,6 +191,28 @@ Using Autotools:
 3. To run manually: `./tests/functionaltests/functionaltests`
 4. To run a specific test manually: `CMOCKA_TEST_FILTER=test_name ./tests/functionaltests/functionaltests`
 
+### Writing functional tests
+Functional tests simulate real world usage by running Profanity and interacting with it. They verify Profanitys responses to an XMPP server.
+
+They are located in `tests/functionaltests/`.
+
+#### Helper Functions
+We provide a set of helper functions:
+- `prof_input(const char* input)`: Sends a command or message to Profanity.
+- `prof_output_regex(const char* regex)`: Verifies that Profanity's output matches the given regular expression.
+- `prof_output_exact(const char* text)`: Verifies that Profanity's output exactly matches the given text.
+- `prof_connect()`: Connects Profanity to the simulated stabber server.
+- `stbbr_for_id(const char* id, const char* xml)`: Tells stabber to respond with `xml` when it receives a stanza with the given `id`.
+- `stbbr_for_query(const char* xmlns, const char* xml): Tells the server to respond with xml whenever a query for a specific namespace is received.
+
+To see the complete list check `proftest.h`.
+
+#### Adding a new test
+1. Create a new test file (`test_new_feature.c` and `test_new_feature.h`) in `tests/functionaltests/`.
+2. Define your test functions using the `void test_name(void** state)` signature.
+3. Use the `PROF_FUNC_TEST(test_name)` macro to register your test in `tests/functionaltests/functionaltests.c`. This macro automatically handles the setup (`init_prof_test`) and teardown (`close_prof_test`) for each test.
+4. Add the new source file to `functionaltest_sources` in both `meson.build` and `Makefile.am`.
+
 ### Valgrind
 We provide a suppressions file `prof.supp`. It is a combination of the suppressions for shipped with glib2, python and custom rules.