feat(mods): add README file

mod/README.md

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+## Etcd modules
+
+etcd modules (mods) are higher order pieces of functionality that only
+speak to the client etcd API and are presented in the `/mod` HTTP path
+of the etcd service.
+
+The basic idea is that etcd can ship things like dashboards, master
+election APIs and other helpful services that would normally have to
+stand up and talk to an etcd cluster directly in the binary. It is a
+convienence and hopefully eases complexity in deployments.