Update README

I added a note about enabling SplunkWeb, referencing attributes for more options.

I suspect most people who install the server are going to want this, and as a new user of the cookbook, my impression based on the README was that only recipe[splunk::server] is needed.  It took some digging to find the template for the applicable Splunk configuration, then trace back to an if block controlled by an attribute indicating that the server is a dedicated search head which is set only if the node has a particular role assigned.