add configuration to use react-express-mongo sample with Docker Dev Environments feature (#271)

Signed-off-by: Guillaume Lours <guillaume.lours@docker.com>

react-express-mysql/README.md

@@ -97,3 +97,14 @@ Removing react-express-mysql_db_1       ... done
 Removing network react-express-mysql_default
 
 ```
+
+## Use with Docker Development Environments
+
+You can use this sample with the Dev Environments feature of Docker Desktop.
+
+![Screenshot of creating a Dev Environment in Docker Desktop](../dev-envs.png)
+
+To develop directly on the services inside containers, use the HTTPS Git url of the sample:
+```
+https://github.com/docker/awesome-compose/tree/master/react-express-mongodb
+```