design: add go2draft-type-parameters.md
design: add go2draft-type-parameters.md

For golang/go#15292

Change-Id: I3c1dc1ffbc2e5df958f93dcb0d7a89a88f6b5107
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/238003
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/proposal,golang/proposal