Document how to get an API token

[machristie]

The steps are:

1. Log into CloudLaunch
2. Go to /api/v1/auth/api-token-auth/
3. If token isn't displayed, click the POST button

Note: The code fix for galaxyproject/cloudlaunch#145 is not yet deployed and these instructions won't work on launch.usegalaxy.org until they are.