Docs: Provide curl syntax for retrieving trello listId's

This replaces the requirement to download the cprice404's tool to retrieve the
listId. Primarily to keep the documentation flow simple.

Signed-off-by: Ken Barber <ken@bob.sh>
Docs: change webhook end-point to /event/github

Changing the end-point specified in docs from /event/pull_request to
/event/github as the old URL is now unsupported.

Signed-off-by: Ken Barber <ken@bob.sh>
Docs: Include note about account verification

If you don't verify you're account earlier you'll get an error later. Better to
warn people now when they are setting up Heroku.

Signed-off-by: Ken Barber <ken@bob.sh>