benchmarks: document required Linux packages

To get the perf tool to work on Linux, you need to install
some packages. Document them in the README.

Change-Id: I00333d94e1eb8fccddf4cdcdd69cb3156b71f40b
Reviewed-on: https://go-review.googlesource.com/73878
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed