Adding minimal VirtualHost config for mod_rewrite

I tried to set up kirby on a local VirtualHost for development and all
urls except '/' returned 404. I racked my brain for a while when I
couldn't find a reference to the possible issue in your docs or readme.

Once I figured out that Rewrite directives were being ignored in my
.htaccess file, I thought I would try to save someone else the trouble
by adding a note in your readme.