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/root/README.md

@@ -0,0 +1,22 @@
+# root
+
+*root* simply specifies the root of where CoreDNS finds (e.g.) zone files.
+
+The default root is the current working directory of CoreDNS. A relative root path is relative to
+the current working directory.
+
+## Syntax
+
+~~~ txt
+root PATH
+~~~
+
+**PATH** is the directory to set as CoreDNS' root.
+
+## Examples
+
+Serve zone data (when the *file* plugin is used) from `/etc/coredns/zones`:
+
+~~~ txt
+root /etc/coredns/zones
+~~~