Add note about unrecognized extras

Author: astrofrog

peps/pep-0771.rst

@@ -229,6 +229,13 @@ Note that ``package[]`` would continue to be equivalent to ``package`` and would
 not be provided as a way to install without default extras (see the `Rejected
 Ideas`_ section for the rationale).
 
+We also note that some tools such as `pip`_ currently ignore unrecognized
+extras, and emit a warning to the user to indicate that the extra has not been
+recognized. In this case, if no extras are recognized as being valid in a
+dependency specification, we recommend that this case be treated as if the user
+had not passed any explicit extras, and therefore any default extras should be
+installed.
+
 Installing without default extras
 ---------------------------------