doc: update README file
doc: update README file

Change-Id: I97ae0f692d8237f4f410298740371991c18caaf8
Reviewed-on: https://go-review.googlesource.com/1341
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/benchmarks,nickng/go-benchmarks,golang/benchmarks