docs(proxy): add documentation for corporate proxy configuration with backend proxy (angular#4041) (angular#4070)

Fix angular#4041

Author: sinedied

docs/documentation/stories/using-corporate-proxy.md

@@ -0,0 +1,42 @@
+# Using corporate proxy
+
+If you work behind a corporate proxy, the regular [backend proxy](http://github.com/angular/angular-cli#proxy-to-backend) configuration will not work if you try to proxy calls to any URL outside your local network.
+
+In this case, you can configure the backend proxy to redirect calls through your corporate proxy using an agent:
+
+```bash
+npm install --save-dev https-proxy-agent
+```
+
+Then instead of using a `proxy.conf.json` file, we create a file called `proxy.conf.js` with the content
+
+```js
+var HttpsProxyAgent = require('https-proxy-agent');
+var proxyConfig = [{
+  context: '/api',
+  target: 'http://your-remote-server.com:3000',
+  secure: false
+}];
+
+function setupForCorporateProxy(proxyConfig) {
+  var proxyServer = process.env.http_proxy || process.env.HTTP_PROXY;
+  if (proxyServer) {
+    var agent = new HttpsProxyAgent(proxyServer);
+    console.log('Using corporate proxy server: ' + proxyServer);
+    proxyConfig.forEach(function(entry) {
+      entry.agent = agent;
+    });
+  }
+  return proxyConfig;
+}
+
+module.exports = setupForCorporateProxy(proxyConfig);
+```
+
+and edit the `package.json` file's start script accordingly
+
+```json
+"start": "ng serve --proxy-config proxy.conf.js",
+```
+
+This way if you have a `http_proxy` or `HTTP_PROXY` environment variable defined, an agent will automatically be added to pass calls through your corporate proxy when running `npm start`.