[DOCS][Dev] Add Debugging and Testing

docs/developer/src/debugging.md

@@ -0,0 +1,34 @@
+Debugging and Testing
+=====================
+
+Testing
+-------
+
+GNU social isn't too big or hard to understand, but remember that
+events are asynchronous and every handler must take care not to
+interfere with core and other plugins normal behaviours.
+
+We encourage [Test-driven development](https://en.wikipedia.org/wiki/Test-driven_development),
+as it helps preventing regressions and unexpected behaviour.
+
+To run GNU social's tests you can execute:
+
+```sh
+make tests
+```
+
+To write your own `TestCase` you can `use App\Util\GNUsocialTestCase`.
+
+To mock HTTP requests you can `$client = static::createClient();`.
+
+Finally, to use services such as queues you must `parent::bootKernel();`.
+
+As the test framework we adopted PHPUnit, you have a list of possible assertions
+in [PHPUnit Manual](https://phpunit.readthedocs.io/en/9.5/).
+
+Debugging
+---------
+
+Because we are using Symfony, we recall that a useful tool for debugging
+is [Symfony's VarDumper component](https://symfony.com/doc/current/components/var_dumper.html),
+as a more friendly alternative to PHP's `var_dump` and `print_r`.