Fix doc for windows+graphviz; Fixes #11

Author: parrt

README.md

@@ -58,6 +58,21 @@ $ pip list | grep -i torch
 torch                              1.6.0
 ```
 
+### Graphviz for tsensor.astviz()
+
+For displaying abstract syntax trees (ASTs) with `tsensor.astviz(...)`, then you need the `dot` executable from graphviz, not just the python library. On mac, do
+
+```
+brew install graphviz
+```
+
+On Windows, apparently you need
+
+```
+conda install python-graphviz  # Do this first; get's dot executable and py lib
+pip install tensor-sensor      # Or one of the other installs
+```
+
 
 ## Limitations