Add multiple field instructions to readme

I thought it could be helpful to have instructions on setting up an autocomplete field that accepts multiple values in the readme.  It took some searching this morning but I eventually found the solution in your issues, (issue #20 I believe).  I've added a few lines in the 'Views' section about adding the `'data-delimiter' => ','` and `:multiple => true` options.  I feel that this is a fairly common use case scenario (adding multiple tags to a model object, etc.) so deserves to be in the main documentation.  Great gem btw, very easy to drop in and get working quickly.  Thanks!