readme: add attribution for the Gopher image

Change-Id: I3b1317f0ab46e03d8c5a0af74c83183710a75055
Reviewed-on: https://go-review.googlesource.com/36214
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

README.md

@@ -4,6 +4,7 @@ Go is an open source programming language that makes it easy to build simple,
 reliable, and efficient software.
 
 ![Gopher image](doc/gopher/fiveyears.jpg)
+*Gopher image by [Renee French][rf], licensed under [Creative Commons 3.0 Attributions license][cc3-by].*
 
 Our canonical Git repository is located at https://go.googlesource.com/go.
 There is a mirror of the repository at https://github.com/golang/go.
@@ -39,3 +40,6 @@ Note that the Go project does not use GitHub pull requests, and that
 we use the issue tracker for bug reports and proposals only. See
 https://golang.org/wiki/Questions for a list of places to ask
 questions about the Go language.
+
+[rf]: https://reneefrench.blogspot.com/
+[cc3-by]: https://creativecommons.org/licenses/by/3.0/