No information about neccesary libraries in README.

While trying to compile and flash to Arduino Nano clone i have some problems with finding proper libriaries.
I think it will be nice to write hint for future users what libriaries are neccesary and how to install them.
For example what i was neccesary to do on Ubuntu 18.04:
```
sudo apt install gcc-avr avr-libc avrdude
```

For less technical people is not so obvious :)