Added a README

elasticsearch/README.md

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+This Docker image provides an easily configurable Elasticsearch node. Via
+port mappings, it is becomes easy to create an arbitrarily sized cluster of
+nodes. As long as the versions match, you can mix-and-match "real"
+Elasticsearch nodes with containerized ones.
+
+# Basic Usage
+
+To start an Elasticsearch data node that listens on the standard ports on
+your host's network interface:
+
+    docker run -d -p 9200:9200 -p 9300:9300 itzg/elasticsearch
+
+You'll then be able to connect to the Elasticsearch HTTP interface to confirm
+it's alive:
+
+http://DOCKERHOST:9200/
+
+    {
+      "status" : 200,
+      "name" : "Charon",
+      "version" : {
+        "number" : "1.3.5",
+        "build_hash" : "4a50e7df768fddd572f48830ae9c35e4ded86ac1",
+        "build_timestamp" : "2014-11-05T15:21:28Z",
+        "build_snapshot" : false,
+        "lucene_version" : "4.9"
+      },
+      "tagline" : "You Know, for Search"
+    }
+
+Where `DOCKERHOST` would be the actual hostname of your host running
+Docker.
+
+# Basic multi-node cluster
+
+Running a multi-node cluster (3-node in this example) is almost as easy:
+
+    docker run -d -p 9200:9200 -p 9300:9300 itzg/elasticsearch
+    docker run -d -p 9201:9200 -p 9301:9300 itzg/elasticsearch
+    docker run -d -p 9202:9200 -p 9302:9300 itzg/elasticsearch
+
+where the only difference was the host port binding of `9200:`/`9300:`,
+`9201:`/`9301:`, and `9202:`/`9302:`. By default, Elasticsearch uses
+[Zen Discovery](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/modules-discovery-zen.html), so the three nodes find each other and form a cluster. You
+can confirm that by checking the cluster health for the presence of
+three nodes (`number_of_nodes`):
+
+http://DOCKERHOST:9200/_cluster/health?pretty
+
+    {
+      "cluster_name" : "elasticsearch",
+      "status" : "green",
+      "timed_out" : false,
+      "number_of_nodes" : 3,
+      "number_of_data_nodes" : 3,
+      "active_primary_shards" : 0,
+      "active_shards" : 0,
+      "relocating_shards" : 0,
+      "initializing_shards" : 0,
+      "unassigned_shards" : 0
+    }
+
+# Configuration
+
+The following configuration options are specified using `docker run`
+environment variables (`-e`) like
+
+    docker run ... -e NAME=VALUE ... itzg/elasticsearch
+
+## Cluster Name
+
+If joining a pre-existing cluster, then you may need to specify a cluster name
+different than the default "elasticsearch":
+
+    -e CLUSTER=dockers
+
+## Zen Unicast Hosts
+
+When joining a multi-physical-host cluster, multicast may not be supported
+on the physical network. In that case, your node can reference specific one or more hosts in
+the cluster via the
+[Zen Unicast Hosts](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/modules-discovery-zen.html#unicast) capability as a comma-separated list of `HOST:PORT` pairs:
+
+    -e UNICAST_HOSTS=HOST:PORT[,HOST:PORT]
+
+such as
+
+    -e UNICAST_HOSTS=192.168.0.100:9300
+
+## Plugins
+
+You can install one or more plugins before startup by passing a comma-separated
+list of plugins.
+
+    -e PLUGINS=ID[,ID]
+
+In this example, it will install the Marvel plugin
+
+    -e PLUGINS=elasticsearch/marvel/latest
+
+Many more plugins [are available here](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/modules-plugins.html#known-plugins).
+
+## Publish As
+
+Since the container gives the Elasticsearch software an isolated perspective
+of its networking, it will most likely advertise its published address with
+a container-internal IP address. This can be overridden with a physical networking
+name and port using:
+
+    -e PUBLISH_AS=DOCKERHOST:9301
+
+_Author Note: I have yet to hit a case where this was actually necessary. Other
+than the cosmetic weirdness in the logs, Elasticsearch seems to be quite tolerant._
+
+## Node Name
+
+Rather than use the randomly assigned node name, you can indicate a specific
+one using:
+
+    -e NODE_NAME=Docker