Add information to `DEPLOYMENT.md` about how billing and credits should work

[kklamberty]

The DEPLOYMENT.md file tells students to make a Digital Ocean account without entering billing information and to be sure to use their student developer pack credits, but it's not clear that is still possible or quite what they should do. Once we figure out if there are any helpful links, we should add them to the markdown file to make this more obvious to students.

It's possible they always needed to add billing information to make a droplet, but it has definitely come up (last semester and this semester at least) and caused some confusion. There may be something we can add that will help, or something we could clarify so that they know they will need to add billing information.