README: make it clear that golint is deprecated and frozen

For golang/go#38968.

Change-Id: I52fe4f2934910600c89ac705a7fb3c57b4844a9d
Reviewed-on: https://go-review.googlesource.com/c/lint/+/318189
Trust: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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