message: mavlink_passthrough: note how build plugin
diff: @@ -19,6 +19,9 @@ class MavlinkPassthroughImpl;
  * "With great power comes great responsibility." - This plugin allows
  * you to send and receive MAVLink messages. There is no checking or
  * safe-guards, you're on your own, and you have been warned.
  + *
  + * @note This plugin is not included in the build by default. To add it
  + * `make ENABLE_MAVLINK_PASSTHROUGH=1` is required.
  */
  class MavlinkPassthrough : public PluginBase {
  public:
  