Documentation to state sw use HTTPS only

[dman777]

Please see Documentation needs to state service workers will work only on https. #101 and Caching with platinum-sw-register #106

Description
Service workers only work on HTTPS. They will fail silently by default on HTTP. I say 'default' because without on-service-worker-error the error will not appear on dev console which is the usual convention. For example, Google map api will give getCurrentPosition() and watchPosition() are deprecated on insecure origins warning in console without any handler.

Solution: