Consistent subproject readmes

Per some discussion with @rkneufeld:

* All READMEs have a Description section
* Added missing READMEs to jetty and tomcat
* app-template and service-template don't have install sections since no install is required
* Only app-template and service-template have usage sections. Other app and service usage lives in the generated apps.
* Removed redundant mentions of lein version and sub install since that's covered in the top-level README