add DRY Principle

- add DRY definition and example

README.md

@@ -32,6 +32,7 @@ Laws, Theories, Principles and Patterns that developers will find useful.
     * [The Liskov Substitution Principle](#the-liskov-substitution-principle)
     * [The Interface Segregation Principle](#the-interface-segregation-principle)
     * [The Dependency Inversion Principle](#the-dependency-inversion-principle)
+    * [The DRY Principle](#the-dry-principle)
 * [TODO](#todo)
 
 <!-- vim-markdown-toc -->
@@ -381,6 +382,36 @@ See also:
 - [Inversion of Control](#todo)
 - [Dependency Injection](#todo)
 
+### The DRY Principle
+
+[The DRY Principle on Wikipedia](https://en.wikipedia.org/wiki/Don%27t_repeat_yourself)
+
+> Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.
+
+DRY is an acronym for _Don't Repeat Yourself_. This principle aims to help developers reducing the repetition of code and keep the information in a single place.
+
+> The opposite of DRY would be _WET_ (Write Everything Twice or We Enjoy Typing).
+
+The principle is easy to understand and can be used with any other methodology, design pattern and/or language. It was cited in 1999 by Andrew Hunt and Dave Thomas in the book [The Pragmatic Developer](https://en.wikipedia.org/wiki/The_Pragmatic_Programmer).
+
+In practice, if you have the same piece of code/information in two (or more) different places, you can use the DRY Principle to merge those pieces into a single one and reuse it wherever you want/need. The DRY principle can be used for codes, documentations, schemas, build systems...
+
+Besides avoiding the repetition, DRY help developers to deliver maintainable, readable and reusable code. It also helps developers to keep the testing step easier - in this specific case, DRY keep unit and integration tests simple. Because if the code isn't repeated, you just need to test it a single time.
+
+In a real-world application, we can translate the use of DRY with this code:
+
+```js
+// check if user is logged in
+function isUserLoggedIn() {
+  const user = context.getUser();
+
+  return typeof user.id !== null;
+}
+```
+
+In the example above, to check if the user is logged in we can import this function to anywhere and get the result - without DRY, you would write it every time you need to know if the user is logged in.
+
+
 ## TODO
 
 Hi! If you land here, you've clicked on a link to a topic I've not written up yet, sorry about this - this is work in progress!