Fix & usage update to README

Great package!  Its working well for me.  

Two updates in this pull request, based on my usage of the package by going through the README.

1) Fix the buildpack url in the .buildpacks file to be gregburek/heroku-buildpack-pgbouncer instead of heroku/heroku-buildpack-pgbouncer, which doesn't exist.
2) Added a note about the PGBOUNCER_URI, which I had to dig a bit to figure out since the README doesn't currently mention.