.github: add .md extension to SUPPORT file

This makes GitHub render the markdown file automatically
on their web UI.

SUPPORT.md is the recommended file name according to the GitHub
documentation:
https://help.github.com/articles/adding-support-resources-to-your-project/

Fixes #21223

Change-Id: I9f9b9daced9c29a16850e8c446656f353f50b1ae
Reviewed-on: https://go-review.googlesource.com/52013Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>