Fixed Flutter version

[cintek]

Suggestion: Add a fixed version number of Flutter to the Readme so everyone knows which version is needed to be able to build the application.
With Flutter it was already two times the case that I ran flutter update and then there was a change in the SDK which prevented a successful build. Of course I can change the code so it works again. But then others have to update the SDK again and if there is an even newer version of Flutter in the meantime they probably install that and everything starts again ;)

What do you think about this? Do you have the same experience?

[EdGeraghty]

See #794