doc: 2020 is the Year of the Gopher

Starting with 2014 (golang.org/cl/46660043), we have enjoyed 6
consecutive years of the gopher. Now, the slice¹ of gophers is
ready to make its way into the next decade, as 2020 is the new
Year of the Gopher.

¹ https://en.wikipedia.org/w/index.php?title=List_of_English_terms_of_venery,_by_animal&oldid=932675028#G

Change-Id: I5f9598dbedb373bd13021964193fa9e44c67693e
Reviewed-on: https://go-review.googlesource.com/c/go/+/213017


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Symonds <dsymonds@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>