Remove README note about Heroku access

Suspenders' `./bin/setup` script is smart enough to handle this case.
The `heroku join` lines are protected by `if` statements:

```bash
if heroku join --app trailmix-staging &> /dev/null; then

if heroku join --app trailmix-production &> /dev/null; then
```

The failing branch of the conditionals print messages to `stdout`:

> Ask for access to the "trailmix-staging" Heroku app
> Ask for access to the "trailmix-production" Heroku app

https://github.com/thoughtbot/suspenders