apidiff: changes to README
apidiff: changes to README

Changes in response to final comments on https://go-review.googlesource.com/c/exp/+/143597.

Change-Id: I29ce24d71c0758aee4e9b4a01f990fb6eed40ffa
Reviewed-on: https://go-review.googlesource.com/c/149037
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/exp,golang/exp,golang/exp