add comment about why shell=bash is needed

.vimrc

@@ -29,6 +29,7 @@ set switchbuf=useopen
 set numberwidth=5
 set showtabline=2
 set winwidth=79
+" This makes RVM work inside Vim. I have no idea why.
 set shell=bash
 " Prevent Vim from clobbering the scrollback buffer. See
 " http://www.shallowsky.com/linux/noaltscreen.html