apidiff: README: describe compatibility between numeric types
apidiff: README: describe compatibility between numeric types

Change-Id: Ic672ea87c0f5abf711ccfea33b06f9f0b524c96c
Reviewed-on: https://go-review.googlesource.com/c/150177
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/exp,golang/exp,golang/exp