Ajout installation de docker-compose via apt

HowtoDockerCompose.md

@@ -8,6 +8,13 @@ Docker multi-conteneurs.
 
 ## Installation
 
+~~~
+# apt install docker-compose
+
+$ docker-compose --version
+docker-compose version 1.21.0, build unknown
+~~~
+
 ### Manuellement
 ~~~
 $ curl -L "https://github.com/docker/compose/releases/download/1.18.0/docker-compose-$(uname -s)-$(uname -m)" > /usr/local/bin/docker-compose