Update README with usage instructions

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

README.md

@@ -1,3 +1,12 @@
 # markup
 
-A small, easy markup language for doing basic replacements in text.
+A small, easy markup language for doing basic replacements in text.
+
+# Usage
+
+If you want to use it inside of Python, you should be able to just drop in `markup.py` into your
+Python program.
+
+If you want to use it on the command line, you can either call `./markup.py` and pass your text to
+process through STDIN. Otherwise, you can pass in a (single) file as an argument, e.g.
+`./markup.py filename.txt`