Provide guidance for naming modules

Name a module after the software it provides, not the functionality it
provides.  This allows alternative software providing the same
functionality to be located in Sovereign.

CONTRIBUTING.md

@@ -14,6 +14,10 @@ You'll want to set up a [local development environment](https://github.com/sover
 
 Sovereign is an Ansible playbook that uses the modules in this repository to configure a server. Modules should conform to the following design principles.
 
+### Naming
+
+Modules should be named after the software they add (as opposed to the functionality they provide). Soverign is currently inconsistent on this. For example, there are the `ircbouncer` and `blog` modules, but there are also the `owncloud` and `tarsnap` modules. Please name modules after the software used, though, so that it is possible to provide alternatives for functionality.
+
 ### Making decisions
 
 A module exists to make decisions about how a service should be installed and configured. Make these decisions and minimize or eliminate configuration options exposed to the user. When in doubt, make a decision, and if the community feedback is vocal enough, only then expose an option.