Add bin/repository clusterize and document setup and migration for clusters

Summary: Ref T4292. This provides at least some sort of hint about how to set up cluster repositories.

Test Plan:

• Read documentation.
• Ran bin/repository clusterize to add + remove clusters.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4292

Differential Revision: https://secure.phabricator.com/D15798