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>

README.md

diff --git a/README.md b/README.md
index ba4956f..989b1dc 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,7 @@
+**NOTE:** Golint is [deprecated and frozen](https://github.com/golang/go/issues/38968).
+There's no drop-in replacement for it, but tools such as [Staticcheck](https://staticcheck.io/)
+and `go vet` should be used instead.
+
 Golint is a linter for Go source code.
 
 [![Go Reference](https://pkg.go.dev/badge/golang.org/x/lint.svg)](https://pkg.go.dev/golang.org/x/lint)