Add documentation to include images in exercises.

Author: CJs0800

demo-repository/exercises/demo/descr.html

@@ -3,7 +3,7 @@
 </p>
 
 <h2>The task</h2>
-
+<img src="/static/demo/images/OCaml.png">
 <p>
   In this test-bed exercise you are asked to (re)implement the basic
   integer-arithmetic functions.

docs/howto-include_images_in_exercises.md

@@ -0,0 +1,19 @@
+How to include our own static content to an Exercise Statement
+==============================================================
+
+## Context
+
+If you create an exercise statement, you probably want to add your own static content such as images in an exercise statement. 
+
+`LearnOCaml` provides a feature to include static contents in an exercise statement. In addition to always being able to include a link to external content, it's possible to include specific static content. 
+
+## How-to
+
+Lets see how to include `OCaml.png` in exercise `demo`.
+Follow these steps:
+
+1. Create a directory `<repo>/exercices/demo/images`.
+2. Put `OCaml.png` in there.
+3. In `<repo>/exercises/demo/decr.html` or `<repo>/exercises/demo/descr.md`, use the following syntaxe <br> `<img src="/static/demo/images/OCaml.png">`. 
+
+To do this we can't use relative URLs.