testing: document MainStart signature change in release notes

[bradfitz]

c56cc9b (https://golang.org/cl/32455) changed the signature of testing.MainStart

from:
https://golang.org/pkg/testing/#MainStart

func MainStart(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample) *M

To:
https://beta.golang.org/pkg/testing/#MainStart

func MainStart(deps testDeps, tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample) *M

This is not in our release notes, and just bit Kubernetes in kubernetes/kubernetes#38926 (comment)

For the record, this does say:

It is not meant to be called directly and is not subject to the Go 1 compatibility document. It may change signature from release to release.

/cc @rsc