middleware/root: add it (#330)

This PR adds the *root* middleware that specifies a path where
all zone file (the *file* middleware is the only consumer now) can
be found. It works the same as in Caddy.

Documentation can be found in the README.md of the middleware.

Fixes #307

middleware/root/README.md

@@ -0,0 +1,21 @@
+# 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* middleware is used) from `/etc/coredns/zones`:
+
+~~~ txt
+root /etc/coredns/zones
+~~~