[x/blog] content: add go1.15-proposals.article
[x/blog] content: add go1.15-proposals.article

Change-Id: Iece46c6e47ad2ee2ea003d73615792114f0117af
Reviewed-on: https://go-review.googlesource.com/c/blog/+/216199
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
X-Blog-Commit: d1582acb251f89b73212cc0f727df3ab12c230b4

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website