[dev.boringcrypto] README.boringcrypto.md: add note about the demise of the branch

CL 403154 merged the branch back into the standard Go branch.

Fixes #51940.

Change-Id: Ibda89792c01630adce7ee3fbd56edbd780fcf79e
Reviewed-on: https://go-review.googlesource.com/c/go/+/403615
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

README.boringcrypto.md

diff --git a/README.boringcrypto.md b/README.boringcrypto.md
index 54adda6..03f5747 100644
--- a/README.boringcrypto.md
+++ b/README.boringcrypto.md
@@ -1,5 +1,9 @@
 # dev.boringcrypto branch
 
+This branch is no more. `GOEXPERIMENT=boringcrypto` on the standard branch does the job now.
+
+Previous notice:
+
 We have been working inside Google on a fork of Go that uses
 BoringCrypto (the core of [BoringSSL][]) for various crypto
 primitives, in furtherance of some [work related to FIPS 140-2][sp].