Add docs for downward API

docs/downward_api.md

+# Downward API
+
+The downward API allows containers to consume information about the system without coupling to the
+kubernetes client or REST API.
+
+### Capabilities
+
+Containers can consume the following information via the downward API:
+
+*   Their pod's name
+*   Their pod's namespace
+
+### Consuming information about a pod in a container
+
+Containers consume information from the downward API using environment variables.  In the future,
+containers will also be able to consume the downward API via a volume plugin.  The `valueFrom`
+field of an environment variable allows you to specify an `ObjectFieldSelector` to select fields
+from the pod's definition.  The `ObjectFieldSelector` has an `apiVersion` field and a `fieldPath`
+field.  The `fieldPath` field is an expression designating a field on the pod.  The `apiVersion`
+field is the version of the API schema that the `fieldPath` is written in terms of.  If the
+`apiVersion` field is not specified it is defaulted to the API version of the enclosing object.
+
+### Example: consuming the downward API
+
+This is an example of a pod that consumes its name and namespace via the downward API:
+
+```json
+{
+  "apiVersion":"v1beta3",
+  "name": "downward-api-pod",
+  "kind": "Pod",
+  "spec": {
+    "manifest": {
+      "containers": [{
+        "name": "example-container",
+        "image": "gcr.io/google_containers/busybox",
+        "command": [ "sh", "-c", "env" ]
+        "env": [{
+          "name": "POD_NAMESPACE",
+          "valueFrom": {
+            "fieldPath": {
+              "apiVersion": "v1beta3",
+              "fieldPath": "metadata.namespace"
+            }
+          }
+        },
+        {
+          "name": "POD_NAME",
+          "valueFrom": {
+            "fieldPath": {
+              "apiVersion": "v1beta3",
+              "fieldPath": "metadata.name"
+            }
+          }
+        }]
+      }]
+    }
+  }
+}]
+```