Add links to whoami plugin reading material (#1815)

* Add links to whoami plugin reading material

* create See Also section

plugin/whoami/README.md

@@ -49,3 +49,10 @@ When queried for "example.org A", CoreDNS will respond with:
 example.org.            0       IN      A       10.240.0.1
 _udp.example.org.       0       IN      SRV     0 0 40212
 ~~~
+
+## See Also
+
+[Read the blog post][blog] on how this plugin is built, or [explore the source code][code].
+
+[blog]: https://coredns.io/2017/03/01/how-to-add-plugins-to-coredns/
+[code]: https://github.com/coredns/coredns/blob/master/plugin/whoami/