README: make it clear that golint is deprecated and frozen
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 <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/lint