added docker readme

contrib/docker/README.md

@@ -0,0 +1,20 @@
+
+## Requirement
+
+The `Dockerfile` script requires a version of `docker` >= 17.05
+
+## Installing docker
+
+The officiel docker install docs use a ppa with a modern version available:
+https://docs.docker.com/install/linux/docker-ce/ubuntu/
+
+## How to run
+
+`docker build -t zstd .`
+
+## test
+
+```
+echo foo | docker run -i --rm zstd | docker run -i --rm zstd zstdcat
+foo
+```