message: README: specify C++11
diff: The [Dronecode SDK](https://www.dronecode.org/sdk/) (previously known as "DroneCore") is an API and library for the [PX4 flight stack](http://github.com/PX4/Firmware) using [MAVLink](https://mavlink.io/en/).
  -It is written in C++ and aiming to be:
  +It is written in C++11 and aiming to be:
  - Easy to use with a simple API.
  - Fast and lightweight.
  