reference opam issue for pin+upgrade combination

prepare-opam.sh

@@ -87,7 +87,7 @@ echo
 
 # Pin build-dep and install everything.
 warn "[prepare-opam] Pinning and installing build-dep and upgrading everything"
-opam pin add -y -n build-dep/ # we have to do this first, or `opam upgrade` cannot pick it up
+opam pin add -y -n build-dep/ # we have to do this first, or `opam upgrade` cannot pick it up, see opam issue #3613.
 # We would really like to do these two in one transaction, but that does not work -- see opam issue #3737.
 # Also, we have to list the packages we pinned explicitly here, or else opam fails to upgrade them if they
 # are pinned to a git branch (it ignores changes in the git branch) -- see opam issue #3727.