Update README.md

Author: thautwarm

README.md

@@ -8,4 +8,6 @@
 npm install -g purescript
 npm install -g spago
 pip install git+https://github.com/purescript-python/installer
-```
+```
+
+NOTE: installing this package sometimes takes a long time, because we will download  the binary files and invoke 2 `setup.py install`.