message: doc: Add more info to spec changelog
diff: - Messages Added:
  - ScalarCmd
  - - ScalarReadCmd
  - - ScalarReading
  + - Replaces VibrateCmd and adds ability to easily extend with new actuator types that take a
  + single value.
  + - SensorReadCmd
  + - Replaces Battery/RSSI messages and adds ability to easily extend with new sensor types.
  + - SensorSubscribeCmd
  + - Allows users to receive realtime updates from devices (pressure sensors kegelcizers,
  + accelerometers in toys that have them, etc...)
  + - SensorUnsubscribeCmd
  + - SensorReading
  - Messages Changed:
  - DeviceList/DeviceAdded
  - - Added Message Attributes _FeatureDescriptor_, _ActuatorType_, _StepRange_, _SensorType_
  + - Remove _FeatureCount_, Message Attributes are now an array of attribute objects instead of
  + many fields of arrays that had to be reconstructed. Should reduce bookkeeping.
  + - Added Message Attributes _FeatureDescriptor_, _ActuatorType_, _SensorType_
  + - Added Device Attributes _DisplayName_
  - Messages Deprecated:
  - VibrateCmd
  - Superceded by ScalarCmd. Will still be available via API calls in client APIs, just no longer
  needs to be a specific message in the protocol.
  + - BatteryLevelCmd
  + - Superceded by SensorReadCmd
  + - RSSILevelCmd
  + - Superceded by SensorReadCmd
  + - BatteryLevelReading
  + - Superceded by SensorReading
  + - RSSILevelReading
  + - Superceded by SensorReading
  ## Version 2 (2020-09-28)
  