Add Install Notes

Added install notes. When I first visited this page, it wasn't apparent if this package was available in pip, until I blindly typed the package name into pip hoping it would work. This will make it obvious as soon as someone visits this repo.

README.md

@@ -5,6 +5,13 @@ Flask-HTTPAuth
 
 Simple extension that provides Basic and Digest HTTP authentication for Flask routes.
 
+Installation
+------------
+The easiest way to install this is through pip.
+```
+pip install Flask-HTTPAuth
+```
+
 Basic authentication example
 ----------------------------