Document process to deploy a new subgraph

[frangio]

We will receive requests to support new chains. We should document the process to deploy a new subgraph for those chains.

How do we deal with subgraph updates? Can subgraphs be versioned so that they can be updated asynchronously/progressively? The UI should be version-aware.

Ideally we would be able to accept PRs to integrate subgraphs deployed by contributors. Is there a checksum to validate that the subgraph code is not tampered?