Add a section about system requirements to the README

Add @rinu's suggestion on how to run fast-export on Windows to the
README, this fixes frej#121.

Author: gcb

README.md

@@ -24,6 +24,16 @@ you want to report a security bug. That way the next person having the
 same problem can benefit from the time spent solving the problem the
 first time.
 
+System Requirements
+-------------------
+
+This project depends on Python 2.7 and the Python mercurial package.
+If Python is not installed, install it before proceeding.  The
+mercurial package can be installed with `pip install mercurial`.
+
+If you're on Windows, run the following commands in git bash (Git for
+Windows).
+
 Usage
 -----