Explain SW versioning in README.md

[jeffposnick]

Unless the project were to move to an automated build-time solution, like sw-precache, it's necessary to bump the version number in sw.js in order to trigger the SW update flow.

If a developer makes any local changes to resources and then fails to bump the version, their existing users will be stuck indefinitely with the old resources due to the cache-first strategy that's being used.

Given the importance of understanding versioning, it would be ideal to explicitly call this out in the README.md.