Clarified README.md

1)  The built in provides condition had no explanation of what the condition was.  I added a line to make clear that provides looks at the request's Accept header, i.e. the condition is: if the Accept header contains the specified value, then the route matches.

2) Explaining the send_file helper by saying it "sends files" is no explanation at all.

3) The explanations for the send_file options were a bit disjointed.  Some explanations started with a capital letter, others did not.  Some explanations capitalized the header names, others did not.  I tried to make them more consistent.