content/go1.15-proposals.article: add correction

Change-Id: I7892c28f347fd0b4984778863f0cd4748b7b3928
Reviewed-on: https://go-review.googlesource.com/c/blog/+/217299
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/content/go1.15-proposals.article b/content/go1.15-proposals.article
index 01bfe48..367bc7b 100644
--- a/content/go1.15-proposals.article
+++ b/content/go1.15-proposals.article
@@ -74,6 +74,9 @@
 constant (possibly untyped) result. This is a fully backward-compatible change
 and we propose to make the necessary adjustments to the spec and compilers.
 
+(Correction: We found out after posting that this change is not backward-compatible;
+see [[https://golang.org/issue/28591#issuecomment-579993684][comment]] for details.)
+
 * Timeline
 
 We believe that none of these three proposals are controversial but there’s