Document `heroku-community/nginx` stable alias

[edmorley]

The recommended way to use this buildpack is via the heroku-community/nginx buildpack-registry stable alias, rather than GitHub URL.

However it isn't mentioned in the README (either how to use, or instructions for maintainers for how to publish to it).

[beanieboi]

It's mentioned in the README:
https://github.com/heroku/heroku-buildpack-nginx#existing-app

[edmorley]

Oh! Search returns zero results, strange :-)
https://github.com/heroku/heroku-buildpack-nginx/search?q=heroku-community%2Fnginx

[edmorley]

It was still quite hard to spot - I wonder if the README needs some re-ordering/cleanup?