message: Add basic docs about Signals
  Content courtesy of Tony's Teams post
diff: # @fluidframework/component-runtime-definitions
  -Interface `IComponentRuntime` specify the component developer API.
  +Interface `IComponentRuntime` specifies the component developer API.
  -TODO: The full set of functionality is under review
  +## Capabilities exposed on `IComponentRuntime`
  +
  +_TODO: The full set of functionality is under review_
  - DDS creation and management APIs
  - Container info and states (connection state, documentId, quorum, audience, etc.)
  @@ -11,3 +13,15 @@ TODO: The full set of functionality is under review
  - Snapshotting
  - DeltaManager
  - Blob Management API.
  +
  +### Signals
  +
  +Signals provide a transient data channel for data (any serializable payload)
  +that doesn't need to be persisted in the op stream.
  +Use signals where possible to avoid storing unnecessary ops, e.g. to transmit presence status during a collaborative session.
  +
  +Signals are not persisted, ordered, or guaranteed. If a client is behind, the op state can be behind the signal state.
  +For this reason people usually stick the currentSeq on the signal, so other clients can wait to process if they are behind.
  +
  +You can send a signal via the container or component runtime. The container will emit the signal event on all signals,
  +but a component will emit the signal event only on signals emitted on that component runtime.
  