Add demo plugin as external plugin (#233)

Signed-off-by: Miek Gieben <miek@miek.nl>

content/explugins/demo.md

@@ -0,0 +1,34 @@
++++
+title = "demo"
+description = "*demo* - returns `1.1.1.1` for `172.0.0.0/8` or `127.0.0.0/8` and `8.8.8.8` otherwise"
+weight = 10
+tags = [  "plugin" , "demo" ]
+categories = [ "plugin", "external" ]
+date = "2017-07-25T21:57:00+08:00"
+repo = "https://github.com/coredns/demo"
+home = "https://github.com/coredns/demo/blob/master/README.md"
++++
+
+## Description
+
+This demo plugin showcases the implementation of source IP based service discovery.
+It is intended as a boilerplate code, so that anyone that is interested in coredns
+implementation could build their plugin on top of this demo plugin.
+
+This demo plugin and its full code has been walked through in past KubeCon talks
+
+Note: In order to add a new plugin, an additional step of `make gen` is needed. Therefore,
+to build the coredns with demo plugin the following should be used:
+```
+docker run -it --rm -v $PWD:/v -w /v golang:1.16 sh -c 'make gen && make'
+```
+
+## Syntax
+
+~~~ txt
+demo
+~~~
+
+## Also See
+
+See the [manual](https://coredns.io/manual).