Add Breaking Changes note

[xphonse]

I've been debugging why my 204 responses didn't cleared the form errors in my react application and this is it, but I found it after debugging for a whole day. Could it be mentioned in the Readme that the latest versions of this package requires a certain version of laravel/framerwork, and a reference to that PR? That could help other devs with older Laravel instalations.

Other than that thanks for the awesome package