Changed readme extension and added bullets to output filename prefixes

First of all great tool. For batch processing this just can't be beat. 

I just made a couple minor changes to the README to improve readability. Unfortunately with the ".txt" file extension the README wont' be rendered as Markdown. I simply changed the extension to ".md". I also bulleted the final list so that it rendered the way I believe was intended.

Hope this helps. Keep up the great work. :+1: