typo fix on the README usage section

A minor cleanup.
have middleware 404 responses return an body msg

Just a simple "Not Found" will do.  Mostly to help in development
so you *know* when its a 404 response using the browser.
if digesting specific paths only update for those paths

Before this would wipe out the digests and only add the digests
for the specific paths.  Was missing a test for this so added that
as well.
Merge pull request #22 from redding/kr-usage

Some tweaks based on real usage