doc/README.md: discourage RELNOTE in CLs

Release notes should always be files under doc/next. Make it clear that
this is the only way to add them: RELNOTE markers in CLs are no longer
supported.

Change-Id: I34d77eb876f57b84ecdc7e5ecbf3eb5c91e6fed8
Reviewed-on: https://go-review.googlesource.com/c/go/+/582075
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/doc/README.md b/doc/README.md
index 666e096..c2b3207 100644
--- a/doc/README.md
+++ b/doc/README.md
@@ -4,7 +4,9 @@
 
 ## For developers
 
-Release notes should be added to `next` by editing existing files or creating new files.
+Release notes should be added to `next` by editing existing files or creating
+new files. **Do not add RELNOTE=yes comments in CLs.** Instead, add a file to
+the CL (or ask the author to do so).
 
 At the end of the development cycle, the files will be merged by being
 concatenated in sorted order by pathname. Files in the directory matching the