docs: Add a remark on the markdown badge

Author: erikmd

.github/workflows/build-coq-demo.yml

@@ -56,3 +56,9 @@ jobs:
       with:
         opam_file: './coq-demo.opam'
         custom_image: ${{ matrix.image }}
+
+# Remark:
+# you may want to add the following badge to your README.md:
+# [![CI](https://github.com/$USER/$REPO/workflows/CI/badge.svg?branch=master)](https://github.com/$USER/$REPO/actions?query=workflow%3ACI)
+# after replacing $USER/$REPO to use your project namespace; note also
+# that CI is the workflow name, defined at the beginning of this file.

README.md

@@ -5,6 +5,17 @@
 Demo of:
 
 * [docker-coq-action](https://github.com/erikmd/docker-coq-action),
-* [Docker-Coq](https://hub.docker.com/r/coqorg/coq) images,
+* [docker-coq](https://hub.docker.com/r/coqorg/coq) and
+  [docker-mathcomp](https://hub.docker.com/r/mathcomp/mathcomp) images,
 * and a workflow named [build-coq-demo.yml](./.github/workflows/build-coq-demo.yml),
 * relying on [coq-demo.opam](./coq-demo.opam)
+
+Note: you can add a badge in your `README.md`, like that of this demo:
+
+```
+[![CI](https://github.com/$USER/$REPO/workflows/CI/badge.svg?branch=master)](https://github.com/$USER/$REPO/actions?query=workflow%3ACI)
+```
+
+after replacing `$USER/$REPO` to use your project namespace; note also
+that CI is the workflow name, defined at the beginning of [this file](
+./.github/workflows/build-coq-demo.yml).