move documentation cloud.gov setup to readme

[amirbey]

User Story

An user should find the details for setting up in the Federalist app repository README instead of the uses docs.

Background (Optional)

Acceptance Criteria

• [ ] migrate omitted contents of https://federalist.18f.gov/documentation/cloud-gov-setup/ to README
• [ ] Change made live via <TODO>.

After evaluating, edit this part:

Level of effort - <low/medium/high>

Implementation outline (if higher than "low" effort):

• [ ] <TODO>