docs: add back link to canonical upstream

This walks back the change I made in 10992d8 ("Remove
references to wincent.com product pages and repo", Jan 19 2016) a little
bit. I wanted to emphasize GitHub because that's where everybody clones
from (or should clone from), but it feels like mentioning the canonical
upstream is technically the right thing to do.

doc/command-t.txt

@@ -480,6 +480,10 @@ the clone URL for which is:
 
   https://github.com/wincent/command-t.git
 
+The canonical upstream is hosted at:
+
+  https://git.wincent.com/command-t.git
+
 Mirrors exist on GitLab and BitBucket, and are automatically updated once per
 hour: