Add docker plugin README

plugin/README.md

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+# Weave network driver extension for Docker
+
+A [plugin](http://docs.docker.com/engine/extend/plugin_api/) to
+integrate [weave Net](http://weave.works/net/) with Docker.
+
+## Setup:
+
+Before you can use this plugin, you must [install weave](https://github.com/weaveworks/weave#installation) and [configure docker with a "cluster store"](https://docs.docker.com/engine/userguide/networking/get-started-overlay/#step-1-set-up-a-key-value-store).
+
+    $ weave launch
+    $ weave launch-plugin
+    $ docker network create --driver=weave weave
+
+## Starting a container:
+
+    $ docker run --net=weave -ti ubuntu
+
+## Plugin command-line arguments
+
+ * `--socket=<path>`, which is the socket on which to listen for the
+   [plugin protocol](http://docs.docker.com/engine/extend/plugin_api/). 
+   This defaults to what Docker expects, that is
+   `"/run/docker/plugins/weave.sock"`, but you may wish to
+   change it, for instance if you're running more than one instance of
+   the driver for some reason.
+
+ * `--log-level=debug|info|warning|error`, which tells the plugin
+   how much information to emit for debugging.
+
+## Points to note
+
+Network plugins require Docker version 1.9 or later.
+
+### Docker needs a "cluster store"
+
+Weave operates as a "globally scoped" libnetwork driver, which means
+libnetwork will assume all networks and endpoints are commonly known
+to all hosts. It does this by using a shared database which you must
+provide.
+
+As a consequence, you need to supply Docker with the address of a "cluster
+store" when you start it; for example, an etcd installation.
+
+There's no specific documentation for using a cluster store, but the
+first part of [this guide](https://github.com/docker/docker/blob/master/docs/userguide/networking/get-started-overlay.md) may help.
+
+### WeaveDNS not supported
+
+Weave's built-in service discovery mechanism is not currently
+supported by the plugin.  However, Docker provides a rudimentary
+discovery mechanism by writing all user-provided container names and
+hostnames into every container's `/etc/hosts`.
+
+### Restarting
+
+We start the plugin with a restart policy of 'always', because
+Docker attempts to recreate networks on startup which means the plugin
+container has to be started. If it isn't started automatically by Docker
+with a restart policy, it's not possible to start it manually because
+Docker is blocked.