Added lemma about gradual typing

Author: lizmat

doc/Language/glossary.pod6

@@ -477,6 +477,13 @@ X<|FTFY>
 
 Fixed That For You
 
+=head2 gradual typing
+X<|gradual typing>
+
+You don't have to specify types of variables and parameters, but if you do,
+it helps in early determination of impossible dispatches and better
+optimization.  See also L<https://en.wikipedia.org/wiki/Gradual_typing>.
+
 =head2 IIRC
 X<|IIRC>