WIP: autopath as middleware (#859)

autopath as middleware

middleware/autopath/README.md

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+# autopath
+
+The *autopath* middleware allows CoreDNS to perform server side search path completion.
+If it sees a query that matches the first element of the configured search path, *autopath* will
+follow the chain of search path elements and returns the first reply that is not NXDOMAIN.
+On any failures the original reply is returned.
+
+Because *autopath* returns a reply for a name that wasn't the original question it will add a CNAME
+that points from the original name (with the search path element in it) to the name of this answer.
+
+## Syntax
+
+~~~
+autopath [ZONE..] RESOLV-CONF
+~~~
+
+* **ZONES** zones *autopath* should be authoritative for.
+* **RESOLV-CONF** points to a `resolv.conf` like file or uses a special syntax to point to another
+  middleware. For instance `@kubernetes`, will call out to the kubernetes middleware (for each
+  query) to retrieve the search list it should use.
+
+Currently the following set of middleware has implemented *autopath*:
+
+* *kubernetes*
+
+## Examples
+
+~~~
+autopath my-resolv.conf
+~~~
+
+Use `my-resolv.conf` as the file to get the search path from. This file only needs so have one line:
+`search domain1 domain2 ...`
+
+~~~
+autopath @kubernetes
+~~~
+
+Use the search path dynamically retrieved from the kubernetes middleware.