fixing readme
pass in cluster to responsetimes() otherwise you would get this error:
ode.js:134
        throw e; // process.nextTick error, or error event on first tick
        ^
TypeError: Cannot read property repl of undefined
    at /Users/phuesler/projects/wooga/castle/node_modules/cluster-responsetimes/lib/cluster-responsetimes.js:13:10
Merge pull request #1 from phuesler/master

fix readme