Document usage & publishing steps for buildpack registry

[edmorley]

This buildpack is published to the buildpack registry:

$ h buildpacks:versions heroku-community/static
Version  Released At               Status
───────  ────────────────────────  ─────────
1        2019-09-18T17:53:50.634Z  published
         2019-09-18T17:53:50.637Z  pending

It would be good to:

1. Have the README list the heroku-community/static alias as the recommended way to add the buildpack (not the git URL)
2. Add publishing steps to remind people making changes to publish them to the registry

[edmorley]

(1) was fixed by #194.

(2) still needs resolving.