Added Requirement (uber-archive#201)

Just to avoid uber-archive#190

Author: VeeeneX

Diff for: readme.md

@@ -9,6 +9,7 @@ if Harbormaster is not enabled).
 
 Table of Contents
 =================
+* [Requirements](#requirements)
 * [Configuration](#configuration)
   * [Phabricator Configuration](#phabricator-configuration)
   * [Jenkins Setup](#jenkins-setup)
@@ -21,6 +22,12 @@ Table of Contents
 * [Development](#development)
 * [Testing](#testing)
 
+
+Requirements
+=============
+
+* [Arcanist](https://github.com/phacility/arcanist) is installed on the Jenkins nodes where the tests will be run (e.g. not just the master) the `arc` binary is in `$PATH` or configured explicitly in the global settings.
+
 Configuration
 =============