readme: add attribution for the Gopher image
readme: add attribution for the Gopher image

Change-Id: I3b1317f0ab46e03d8c5a0af74c83183710a75055
Reviewed-on: https://go-review.googlesource.com/36214
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

josharian/go,codestation/go,derekmarcotte/go,cloudflare/go,CAFxX/go,codestation/go,vsekhar/elastic-go,golang/go,CAFxX/go,codestation/go,linux-on-ibm-z/go,vsekhar/elastic-go,Cofyc/go,linux-on-ibm-z/go,Cofyc/go,ImJasonH/go,josharian/go,nawawi/go,josharian/go,cloudflare/go,golang/go,golang/go,ImJasonH/go,cloudflare/go,polyverse-security/go,golang/go,codestation/go,CAFxX/go,polyverse-security/go,josharian/go,ImJasonH/go,derekmarcotte/go,golang/go,polyverse-security/go,Cofyc/go,polyverse-security/go,derekmarcotte/go,golang/go,codestation/go,linux-on-ibm-z/go,vsekhar/elastic-go,cloudflare/go,vsekhar/elastic-go,derekmarcotte/go,vsekhar/elastic-go,nawawi/go,Cofyc/go,derekmarcotte/go,akutz/go,josharian/go,derekmarcotte/go,akutz/go,linux-on-ibm-z/go,ImJasonH/go,akutz/go,polyverse-security/go,cloudflare/go,linux-on-ibm-z/go,josharian/go,ImJasonH/go,ImJasonH/go,Cofyc/go,vsekhar/elastic-go,josharian/go,polyverse-security/go,codestation/go,linux-on-ibm-z/go,ImJasonH/go,akutz/go,akutz/go,linux-on-ibm-z/go,Cofyc/go,nawawi/go,nawawi/go,CAFxX/go,derekmarcotte/go,CAFxX/go,vsekhar/elastic-go,christopher-henderson/Go,vsekhar/elastic-go,cloudflare/go,Cofyc/go,CAFxX/go,ImJasonH/go,linux-on-ibm-z/go,derekmarcotte/go,codestation/go,polyverse-security/go,CAFxX/go,josharian/go,akutz/go,cloudflare/go,Cofyc/go,nawawi/go,akutz/go,golang/go,akutz/go,cloudflare/go,nawawi/go,polyverse-security/go,CAFxX/go,nawawi/go,codestation/go,golang/go,nawawi/go