Expand regex example in Readme

Relates to https://github.com/sinatra/sinatra/issues/917.

Changes one of the regular expression route examples to demonstrate the `/ ... /` syntax and that start-of-string (`^`) and end-of-string (`$`) characters can be used to constrain route matches. 

Adds a comment to the other regex example, using Ruby's `%r` syntax, to remind the user that it will match _around_ the expression, which may be unexpected (English only).