[doc] Add unit test requirement for contrib

doc/source/dev_overview.rst

@@ -47,6 +47,8 @@ Setup the local copy of the code
 
    **Make small commits with explicit messages about why you do things as you progress** (``git add`` and ``git commit``).
 
+   **Cover your code with unit tests and run them before submitting your contribution.**
+
 .. new line
 | 3. Submit your contribution: