docs: Update index.md

Follows-up: d3bc210

Author: erikmd

README.md

@@ -17,6 +17,7 @@ Howtos
 
 * [How to set up an environment to develop exercises?](https://github.com/ocaml-sf/learn-ocaml/blob/master/docs/howto-setup-exercise-development-environment.md)
 * [How to write exercises?](https://github.com/ocaml-sf/learn-ocaml/blob/master/docs/howto-write-exercises.md)
+* [How to include images in exercises?](https://github.com/ocaml-sf/learn-ocaml/blob/master/docs/howto-include-images-in-exercises.md)
 * [How to submit an exercise to the global corpus?](https://github.com/ocaml-sf/learn-ocaml/blob/master/docs/howto-submit-an-exercise.md)
 * [How to deploy an instance of Learn OCaml?](https://github.com/ocaml-sf/learn-ocaml/blob/master/docs/howto-deploy-a-learn-ocaml-instance.md)
 * [How to deploy Learn-OCaml statically?](https://github.com/ocaml-sf/learn-ocaml/blob/master/docs/howto-deploy-learn-ocaml-statically.md)

docs/index.md

@@ -17,6 +17,7 @@ Howtos
 
 * [How to set up an environment to develop exercises?](./howto-setup-exercise-development-environment.md)
 * [How to write exercises?](./howto-write-exercises.md)
+* [How to include images in exercises?](./howto-include-images-in-exercises.md)
 * [How to submit an exercise to the global corpus?](./howto-submit-an-exercise.md)
 * [How to deploy an instance of Learn OCaml?](./howto-deploy-a-learn-ocaml-instance.md)
 * [How to deploy Learn-OCaml statically?](./howto-deploy-learn-ocaml-statically.md)