doc: clarify prerequisites in benchmark/README.md

PR-URL: #2034
Reviewed-By: Evan Lucas <[email protected]>
Reviewed-By: Trevor Norris <[email protected]>

Author: Fishrock123

benchmark/README.md

@@ -5,8 +5,12 @@ io.js APIs.
 
 ## Prerequisites
 
-Most of the http benchmarks require [`wrk`][wrk] and [`ab`][ab] being installed.
-These are most often available through your preferred package manager.
+Most of the http benchmarks require [`wrk`][wrk] and [`ab`][ab] (ApacheBench) being installed.
+These may be available through your preferred package manager.
+
+If they are not available:
+- `wrk` may easily built [from source](https://github.com/wg/wrk) via `make`.
+- `ab` is sometimes bundled in a package called `apache2-utils`.
 
 [wrk]: https://github.com/wg/wrk
 [ab]: http://httpd.apache.org/docs/2.2/programs/ab.html