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/secondary/README.md

@@ -0,0 +1,54 @@
+# secondary
+
+*secondary* enables serving a zone retrieved from a primary server.
+
+## Syntax
+
+~~~
+secondary [ZONES...]
+~~~
+
+* **ZONES** zones it should be authoritative for. If empty, the zones from the configuration block
+    are used. Note that without a remote address to *get* the zone from, the above is not that useful.
+
+A working syntax would be:
+
+~~~
+secondary [zones...] {
+    transfer from ADDRESS
+    transfer to ADDRESS
+    upstream ADDRESS...
+}
+~~~
+
+* `transfer from` specifies from which address to fetch the zone. It can be specified multiple times;
+    if one does not work, another will be tried.
+* `transfer to` can be enabled to allow this secondary zone to be transferred again.
+* `upstream` defines upstream resolvers to be used resolve external names found (think CNAMEs)
+  pointing to external names. This is only really useful when CoreDNS is configured as a proxy, for
+  normal authoritative serving you don't need *or* want to use this. **ADDRESS** can be an IP
+  address, and IP:port or a string pointing to a file that is structured as /etc/resolv.conf.
+
+## Examples
+
+Transfer `example.org` from 10.0.1.1, and if that fails try 10.1.2.1.
+
+~~~ corefile
+example.org {
+    secondary {
+        transfer from 10.0.1.1
+        transfer from 10.1.2.1
+    }
+}
+~~~
+
+Or re-export the retrieved zone to other secondaries.
+
+~~~ corefile
+. {
+    secondary example.net {
+        transfer from 10.1.2.1
+        transfer to *
+    }
+}
+~~~