Docs: Cluster Management

[WenyXu]

We need to write the documents for cluster management, and I highly recommend you to have a looking at the following codes.

• The HTTP endpoints of cluster management can be found at here, which may help write documents.
• Join / Remove implementations Currently, we still don't support the management cluster via gRPC. It will be implemented later.

[casbin-bot]

@WenyXu @nodece

[harshalmittal4]

Hi @WenyXu , I'm a beginner at contributing to this repo and would like to try adding the documentation. Please tell me if this needs to be added to the Readme or in some other docs. The Readme contains the HTTP endpoints of cluster management, but not the Join/ Remove implementations.