add Instance ID -> FCM getToken migration guide on changelog

[efstathiosntonas]

@mikehardy it would be nice to add the migration link to CHANGELOG and/or docs, will save some time digging for it: https://firebase.google.com/docs/projects/manage-installations#fid-iid

[mikehardy]

great idea thanks for fishing it out! I just made the change

[efstathiosntonas]

Thanks @mikehardy keep it up 😀