DOC: Mention sounddevice_build.py in CONTRIBUTING.rst

Author: mgeier

CONTRIBUTING.rst

@@ -24,6 +24,10 @@ If you prefer, you can also replace the last command with::
 
 ... where ``-e`` stands for ``--editable``.
 
+Whenever the file ``sounddevice_build.py`` changes (either because you edited it
+or it was updated by pulling from Github), you have to run the last command
+again.
+
 If you used the ``--recursive`` option when cloning, the dynamic libraries for
 *macOS* and *Windows* should already be available.
 If not, you can get the submodule with::