Added docs

Author: durkmurder

docs/RecoverableRandomBeaconStateMachine.md

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+The `storage.RecoverableRandomBeaconStateMachine` formalizes the life-cycle of the Random Beacon keys for each epoch. On the happy path, each consensus participant for the next epoch takes part in a DKG to obtain a threshold key to participate in Flow's Random Beacon. After successfully finishing the DKG protocol, the node obtains a random beacon private key, which is stored in the database along with DKG current state `flow.DKGStateCompleted`. If for any reason the DKG fails, then the private key will be `nil` and DKG current state is set to `flow.DKGStateFailure`.
+In case of failing Epoch switchover, the network goes into Epoch Fallback Mode [EFM]. The governance committee can recover the network via a special EpochRecover transaction. In this case, the set of threshold keys is specified by the governance committee.
+The current implementation focuses on the scenario, where the governance committee re-uses the threshold key set from the last successful epoch transition. While injecting other threshold keys into the nodes is conceptually possible and supported, the utilities for this recovery path are not yet implemented.
+
+[diagram](https://drive.google.com/file/d/1UnJLlTIs8IDOIHZhNUhXakeP_5Re10S4/view?usp=sharing)