Add instruction to attach images

This commit updates the README to explain how to attach images to
test case pages.

Signed-off-by: Luis Araujo <luis.araujo@collabora.co.uk>

README.md

@@ -39,6 +39,12 @@ available: `macro_ostree_preconditions` , `macro_install_packages_preconditions`
 and `macro_modules_preconditions`. Please check the tests cases for example about
 how to use them.
 
+## Images
+
+Test cases can contain images. These are attached to a test case page starting a
+line in one of the special directives with the character `@` followed by the image
+name, then this image should be placed under the `renderer/images` directory.
+
 ## Renderer
 
 The main program is located inside the `renderer/` subdirectory and can be directly