Add NeoBundle / Vundle documentation to README

I have added documentation on how to get this plugin working with both Vundle and NeoBundle. I have also switched the README from RST to Markdown as I couldn't get inline code blocks working in RST. (Yes - that's a silly reason to convert the entire file. I was on my lunch break, cut some slack :) )  The conversion to Markdown doesn't have to stay, but I did want to show you the process for getting this working with Vundle and NeoBundle and I assumed that having it written in markdown and easily convertable to rst was better than not having it at all/