design: add optional "Open issues" section to the proposal template
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 <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

golang/proposal,golang/proposal