Updating readme to include minimal virtualhost config for mod_rewrite to work with .htaccess files

A Kirby site's VirtualHost must have the  `AllowOvverride` directive set to include `FileInfo` and `Options` (or be set to `All`) for the site's directory in order for Rewrite directives to take effect in `.htaccess` files.

I scoured your site for hours looking for a clue as to why url rewriting didn't work on my local machine, and didn't find any references to this possible issue.

Making a note of it in your readme for future, frustrated, non-apache-experts trying to figure out why url-rewriting may be broken.

You may want to add a note of it somewhere on the kirby site as well.

Thanks!