adding the short demo of SOP

demos/sop/README.md

@@ -0,0 +1,32 @@
+Steps:
+
+1. Install the web server.
+
+```console
+$ sudo apt install nodejs-legacy
+$ sudo apt install npm
+$ npm config set strict-ssl false
+$ npm install express
+```
+
+2. Run the web server:
+
+Open one terminal window and run:
+
+```console
+$ node server1/server1.js
+```
+
+```console
+$ node server2/server2.js
+```
+
+3. In Firefox, open two tabs, one visits localhost:3000 (this visits server 1), the other visits localhost:4000 (this visits server 2).
+
+4. In server2/server2.js, enable and disable CORS (cross origin resource sharing), via commenting and uncommenting this line:
+
+```console
+res.header('Access-Control-Allow-Origin', 'http://localhost:3000'); // Allow Server 1
+```
+
+Observe the effects of these changes in the server 1 page.