Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/federation/README.md

@@ -0,0 +1,43 @@
+# federation
+
+The *federation* plugin enables
+[federated](https://kubernetes.io/docs/tasks/federation/federation-service-discovery/) queries to be
+resolved via the kubernetes plugin.
+
+Enabling *federation* without also having *kubernetes* is a noop.
+
+## Syntax
+
+~~~
+federation [ZONES...] {
+    NAME DOMAIN
+~~~
+
+* Each **NAME** and **DOMAIN** defines federation membership. One entry for each. A duplicate
+  **NAME** will silently overwrite any previous value.
+
+## Examples
+
+Here we handle all service requests in the `prod` and `stage` federations.
+
+~~~ txt
+. {
+    kubernetes cluster.local 
+    federation cluster.local {
+        prod prod.feddomain.com
+        staging staging.feddomain.com
+    }
+}
+~~~
+
+Or slightly shorter:
+
+~~~ txt
+cluster.local {
+    kubernetes
+    federation {
+        prod prod.feddomain.com
+        staging staging.feddomain.com
+    }
+}
+~~~