Using snap for installing flutter

[guyluz11]

For install flutter section in readme https://github.com/ubuntu-flutter-community/software#install-flutter

we can also recommend simpler and more elegant way of installing by using the snap version

sudo snap install flutter --classic

[Feichtmeier]

The snap version had it's problems when my_application.cc uses stuff that is not provided in the snap. However, I didn't check for months tbh. I'll give it another try :)

[guyluz11]

Thanks

Update me if it is working and I will be happy to commit that.

[Feichtmeier]

@guyluz11 I think you could add them to README.md, as our CI test works for flutter snap.