Implicit requirement : C++ compiler

I tried to run the application but the `pip install -r requirements.txt` fails with error related to `fasttext` :  
```
error: Microsoft Visual C++ 14.0 is required. Get it with "Microsoft Visual C++ Build Tools": http://landinghub.visualstudio.com/visual-cpp-build-tools
```  

It seems that fasttext needs a C++ compiler. We should reflect that on the documentation.