Minor updates to README.md
Minor updates to README.md

Updated the Issues URL because it is returning a 404 (not sure why .../blob/master/... was inserted). 

If comments are sent to public-webapps, please use a [service-workers] Subject: prefix.
chadoliver/ServiceWorker,slightlyoff/ServiceWorker,karolklp/ServiceWorker,slightlyoff/ServiceWorker,chadoliver/ServiceWorker,karolklp/ServiceWorker,karolklp/ServiceWorker,justinleoye/ServiceWorker,mkruisselbrink/ServiceWorker,chadoliver/ServiceWorker,martinthomson/ServiceWorker,zuzusik/ServiceWorker,mkruisselbrink/ServiceWorker,martinthomson/ServiceWorker,justinleoye/ServiceWorker,martinthomson/ServiceWorker,justinleoye/ServiceWorker,zuzusik/ServiceWorker,zuzusik/ServiceWorker