Document supported Ember version

Author: samselikoff

tests/dummy/app/pods/docs/usage/template.md

@@ -1,5 +1,12 @@
 # Usage
 
+## Requirements
+
+To use Addon Docs, your addon must have a `devDependency` of **Ember v2.8** or higher.
+
+Note that your addon can still support older versions of Ember – it's just that you won't be able to run your Addon Docs documentation site against those older versions. (This means you also cannot write acceptance/application tests against your docs sites on older versions of Ember.)
+
+
 ## Installation
 
 ```sh