Clean up some broken links and stale documentation in gopls/README.md,
and add new documentation for the gopls release policy.
Fixesgolang/go#55267
Change-Id: I9c7ed1f1d3949025f3c02edb69b475cf34f214eb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/446863
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>