Add comments to requirements.txt

requirements.txt

@@ -1,3 +1,6 @@
+# This file contains python only requirements to be installed with pip
+# Python pacakges that cannot be installed with pip (e.g. PyQT4) are not included.
+# Usage: pip install -r requirements.txt
 numpy>=1.8
 matplotlib>=1.3.1
 shapely>=1.3