Add 64-bit Python requirement

docs/installation.md

 * Matplotlib
 * numpy
 * Pillow
-* Python (2 or 3)
+* Python (2 or 3; 64bit required)
-To install dependencies, go into the `python` directory and run (depending on your python version):
+To install dependencies, go into the `python` sub-directory of the repositroy, and run (depending on your python version) from the command line:
 
 `pip install .`