doc/gopher: add README

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/79890046

doc/gopher/README

+The Go gopher was designed by Renee French. (http://reneefrench.blogspot.com/)
+The design is licsensed under the Creative Commons 3.0 Attributions license.
+Read this article for more details: http://blog.golang.org/gopher
+