Added docker documentation

src/docs/asciidoc/docker.adoc

@@ -0,0 +1,9 @@
+== Docker image
+
+The Swagger2Markup-CLI has been published as a Docker image on DockerHub. You can pull the image:
+
+`docker pull swagger2markup/swagger2markup`
+
+You can convert a Swagger Spec by running a Docker container:
+
+`docker run --rm -v $(pwd)/in:/tmp/in -v $(pwd)/out:/tmp/out swagger2markup/swagger2markup convert -i /tmp/in/swagger.yaml -c /tmp/in/config.properties -f /tmp/out/swagger`

src/docs/asciidoc/index.adoc

@@ -26,6 +26,8 @@ include::maven_plugin.adoc[]
 
 include::command_line_interface.adoc[]
 
+include::docker.adoc[]
+
 include::demo.adoc[]
 
 include::contributing.adoc[]