Documentation for for interaction with NervesHub and NervesHubWeb

[ConnorRigby]

Right now there is only one FirmwareChannel open between NervesHub and NervesHubWeb. I think as more features are implemented, it's going to be hard to wedge documentation of the messages passed back and forth over the socket and we should consider adopting a common message format/RPC format.

In other apps i usually use a format along the lines of:

@typedoc "Common RPC format"
@type t %__MODULE__{
   kind: atom(),
   args: map()
}

@type firmware_update_rpc :: %__MODULE__{
  kind: :firmware_update,
  args: %{required(:url) => String.t()}
}

@type some_other_rpc :: %__MODULE__{
  kind: :some_other,
  args: %{optional(:data) => number}
}

@typedoc "All possible RPCs that need to be implemented"
@type rpc :: firmware_update_rpc | some_other_rpc