Multiple updates to HEROKU.md

I've added an extra step to explicitly add the production.rb file.

Added S3 CDN instructions with asset sync gem. Users assets will auto sync with S3 on deploy and should significantly help with Heroku stability under load

Added add-on instructions for some load testing services available through Heroku. Hopefully help us hack on performance and stability issues.

Some general houskeeping, fixed h1 h2 muddle and removed bundle exec from foreman commands, as it uses bundler by default.