Add 'Using CORS in Express'

Using-CORS-in-Express.md

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+### Using CORS in Express
+
+Cross-origin resource sharing (CORS) allows AJAX requests to skip the Same-origin policy and access resources from remote hosts.
+In this post I will show you how to enable CORS support in Express. I will also provide some tips to handle common use cases that come up when working with Single Page Applications, like exposing HTTP sessions and custom headers.
+
+### Enabling CORS
+
+The easiest way to get CORS working in Express is by using the cors npm module.
+
+You can simply add it as a dependency:
+```
+npm install --save cors
+And then use it as middleware:
+var express = require('express');
+var cors = require('cors');
+var app = express();
+app.use(cors());
+/* your regular routes go here */
+```
+
+That’s it. CORS is now enabled.
+If you make a request to your app, you will notice a new header being returned:
+
+`Access-Control-Allow-Origin: *`
+
+The Access-Control-Allow-Origin header determines which origins are allowed to access server resources over CORS (the * wildcard allows access from any origin).