Update documentation to reference path matchers

[SuspiciousDuck]

In the documentation, it doesn't say that you can use path matchers with replace. The README file should show something like replace [matcher] [stream | [re] <search> <replace>], to show that path matcher can be used with replace. It also doesn't mention path matchers in the caddyserver.com documentation.

[mholt]

Good point. It is an HTTP handler, so it accepts a matcher token as the first argument in the Caddyfile. We should update the Readme to show this.