docs: Document how to include local images in exercises (#612)

Close #128

Author: CJs0800

demo-repository/exercises/demo/descr.html

@@ -3,7 +3,7 @@
 </p>
 
 <h2>The task</h2>
-
+<img src="/static/demo/files/OCaml.png" alt="OCaml" width="400px">
 <p>
   In this test-bed exercise you are asked to (re)implement the basic
   integer-arithmetic functions.

demo-repository/exercises/demo/files/OCaml.png

@@ -0,0 +1,22 @@
+How to include images in exercises
+==================================
+
+## Context
+
+If you create an exercise, you may want to add your own static content such as images in the exercise description (`descr.html` or `descr.md`).
+
+Learn-OCaml provides this feature, so that you can either add images hosted in an external domain, or images statically served by the Learn-OCaml server itself (which can be handy for summative exams, for example, when remote resources are blocked).
+
+## How-to
+
+In order to include `OCaml.png` in an exercise called `demo`, you just need to:
+
+1. Create a directory `<repo>/exercices/demo/subdirectory`. (The name of the `subdirectory` is free.)
+2. Put `OCaml.png` in there.
+3. In `<repo>/exercises/demo/descr.html` (resp. `<repo>/exercises/demo/descr.md`), use the following syntax:  
+`<img src="/static/demo/subdirectory/OCaml.png" alt="fallback text">` (resp. `![fallback text](/static/demo/subdirectory/OCaml.png)`).
+
+Note that we can't use relative URLs, hence the leading `/`.
+
+If you want to fix the width (or height) of an image in `descr.html` or `descr.md`, you can use the usual HTML syntax:  
+`<img src="/static/demo/subdirectory/OCaml.png" alt="fallback text" width="400px">`