Update supported Docker versions from docker-library/docs

Author: Hans Kristian Flaatten

README.md

@@ -109,9 +109,13 @@ Node.js Docker project.
 
 # Supported Docker versions
 
-This image is officially supported on Docker version 1.8.3.
+This image is officially supported on Docker version 1.9.1.
 
-Support for older versions (down to 1.0) is provided on a best-effort basis.
+Support for older versions (down to 1.6) is provided on a best-effort basis.
+
+Please see [the Docker installation
+documentation](https://docs.docker.com/installation/) for details on how to
+upgrade your Docker daemon.
 
 # People