Add Py3 notice to readme

cknd · web-flow · commit f1c0b1ee9630 · 2019-12-25T18:47:20.000+01:00
diff --git a/README.md b/README.md
@@ -9,7 +9,7 @@ This is a more helpful version of Python's built-in exception message: It shows
 ```bash
 pip install stackprinter
 ```
-
+[It's only for Python 3.](https://github.com/cknd/stackprinter/issues/2#issuecomment-487424967)
 ### Before
 ```
 Traceback (most recent call last):
