message: README: add API links to device
  This adds a link to device and device_plugin_container APIs.
diff: @@ -30,6 +30,8 @@ The library provides both synchronous (blocking) API calls, as well as asynchron
  ## API overview
  - [dronecore](include/dronecore.h): set up connection, discover devices
  +- [device](include/device.h): a device providing access to modules below using ...
  +- [device_plugin_container.h.in](include/device_plugin_container.h.in) which is auto-generated on build.
  - [info](plugins/info/info.h): general info about a device
  - [telemetry](plugins/telemetry/telemetry.h): to receive telemetry data
  