google-api-go-client: add URL for CONTRIBUTORS agreement.
google-api-go-client: add URL for CONTRIBUTORS agreement.

Change-Id: I99f1fc21b04806ebe04caa738bc87d6e496e1d68
Reviewed-on: https://code-review.googlesource.com/3330
Reviewed-by: Michael McGreevy <e70fec140fa6773c6cfd45cb9b436bd6334f4c24@golang.org>

webhost/google-api-go-client,80nashi/google-api-go-client,digideskio/google-api-go-client,googleapis/google-api-go-client,peterkshultz/google-api-go-client,googleapis/google-api-go-client,stephenwithav/google-api-go-client,google/google-api-go-client,likaiwalkman/google-api-go-client,google/google-api-go-client