design: add optional "Open issues" section to the proposal template

Change-Id: Ib00364d8644bda892a142bd3ef439d3b6b68f205
Reviewed-on: https://go-review.googlesource.com/12885
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/design/TEMPLATE.md b/design/TEMPLATE.md
index dab5567..fb3a013 100644
--- a/design/TEMPLATE.md
+++ b/design/TEMPLATE.md
@@ -33,3 +33,7 @@
 [A description of the steps in the implementation, who will do them, and when.
 This should include a discussion of how the work fits into [Go's release cycle](https://golang.org/wiki/Go-Release-Cycle).]
 
+## Open issues (if applicable)
+
+[A discussion of issues relating to this proposal for which the author does not
+know the solution. This section may be omitted if there are none.]