doc/gopher: add model sheet

This should have been added to the repo after Renee's Gophercon
keynote. I will link to it from the FAQ.

Change-Id: I0e5b88690e288827591d27b99420d3a449f7f662
Reviewed-on: https://go-review.googlesource.com/124615Reviewed-by: Andrew Gerrand <adg@golang.org>