Update README

Summary:
- Update README for gentest
- Fix gentest.rb for the right working dir

Reviewed By: emilsjolander

Differential Revision: D4065863

fbshipit-source-id: 5c6bd9d18779c05c1de24c06a83b148889facb6b

gentest/gentest.rb

@@ -4,6 +4,7 @@ require 'fileutils'
 caps = Selenium::WebDriver::Remote::Capabilities.chrome(
   "loggingPrefs"=>{"browser"=>"ALL", "performance"=>"ALL"})
 browser = Watir::Browser.new(:chrome, :desired_capabilities => caps)
+Dir.chdir(File.dirname($0))
 Dir['fixtures/*.html'].each do |file|
   fixture = File.read(file)
   name = File.basename(file, '.*')