chore(content): basic steps of pen testing

Add source for image; also add a link to read more.

README.md

@@ -123,6 +123,10 @@ These are the most used Content Management Systems (CMS). See a complete list [h
     <img src="https://www.tutorialspoint.com/penetration_testing/images/penetration_testing_method.jpg">
 </p>
 
+*Source: [tutorialspoint](https://www.tutorialspoint.com/penetration_testing/index.htm)*
+
+[Read more about pen testing methodology](https://www.tutorialspoint.com/penetration_testing/penetration_testing_method.htm)
+
 ([Table of Contents](#table-of-contents))
 
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