(PE-3401) Added docs for override-webserver-settings! to README.md
(PE-3401) Added docs for override-webserver-settings! to README.md

Added a section for the `override-webserver-settings!` function to the
README.md.

rlinehan/trapperkeeper-webserver-jetty9,puppetlabs/trapperkeeper-webserver-jetty9,fpringvaldsen/trapperkeeper-webserver-jetty9,richardc/trapperkeeper-webserver-jetty9,richardc/trapperkeeper-webserver-jetty9,puppetlabs/trapperkeeper-webserver-jetty9,rlinehan/trapperkeeper-webserver-jetty9,nwolfe/trapperkeeper-webserver-jetty9,fpringvaldsen/trapperkeeper-webserver-jetty9,nwolfe/trapperkeeper-webserver-jetty9,camlow325/trapperkeeper-webserver-jetty9,camlow325/trapperkeeper-webserver-jetty9