doc/gopher: add biplane.jpg
doc/gopher: add biplane.jpg

LGTM=0intro
R=golang-codereviews, 0intro
CC=golang-codereviews
https://codereview.appspot.com/150960043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go