update readme

Author: djih

README.md

@@ -14,7 +14,7 @@ Amplitude-Javascript
   r.onload=function(){t.amplitude.runQueuedFunctions()};var s=e.getElementsByTagName("script")[0];
   s.parentNode.insertBefore(r,s);var i=function(){this._q=[];return this};function o(t){
   i.prototype[t]=function(){this._q.push([t].concat(Array.prototype.slice.call(arguments,0)));
-  return this}}var a=["add","set","setOnce","unset"];for(var u=0;u<a.length;u++){o(a[u]);
+  return this}}var a=["add","append","set","setOnce","unset"];for(var u=0;u<a.length;u++){o(a[u]);
   }n.Identify=i;n._q=[];function c(t){n[t]=function(){n._q.push([t].concat(Array.prototype.slice.call(arguments,0)));
   }}var l=["init","logEvent","logRevenue","setUserId","setUserProperties","setOptOut","setVersionName","setDomain","setDeviceId","setGlobalUserProperties","identify"];
   for(var p=0;p<l.length;p++){c(l[p])}t.amplitude=n})(window,document);