Add a stand-in section for unit tests.

Author: Ivorforce

engine/unit_tests.rst

@@ -3,4 +3,15 @@
 Contributing unit tests
 =======================
 
-See also the `current documentation for unit tests <https://docs.godotengine.org/en/stable/contributing/development/core_and_modules/unit_testing.html>`_.
+The Godot teams are dedicated to ensuring a reliably, bug-free experience for users.
+Therefore, we expect important logic to be tested with unit tests.
+
+Godot's unit tests are built into the binary, and can be run with an argument to the executable.
+For more information, please read the
+`current documentation for unit tests <https://docs.godotengine.org/en/stable/contributing/development/core_and_modules/unit_testing.html>`_.
+
+Unit test guidelines
+--------------------
+
+We do not have dedicated guidelines for unit testing as of yet.
+Please use common best practices for writing unit tests when contributing to Godot.