Add a mention of mypy playground to README (python#6510)

ilevkivskyi · web-flow · commit 0a0e1f4eae98 · 2019-03-15T17:25:35.000-07:00
The playground is quite convenient for quick experimentation and gist sharing. I have been using it sometimes. It makes sense to add a link to README.
diff --git a/README.md b/README.md
@@ -99,6 +99,9 @@ programs, even if they have type errors:
 
     $ python3 PROGRAM
 
+You can also try mypy in an [online playground](https://mypy-play.net/) (developed by
+Yusuke Miyazaki).
+
 [statically typed parts]: http://mypy.readthedocs.io/en/latest/basics.html#function-signatures
 
 
