Document why this is a separate tap

[strugee]

I feel uncomfortable configuring taps that aren't in the Homebrew org, because I trust the Homebrew folks to know their own package manager but I don't trust other people to know Homebrew as well. It would be nice to see some explanation as to why this isn't in the homebrew/homebrew-core repo anymore, i.e. why I should trust Heroku to mess with the state of my package manager.

[strugee]

@mimen closed by accident? https://devcenter.heroku.com/articles/heroku-cli still doesn't mention anything and the README doesn't either