What Changed & Why

The doc at http://ember-cli-deploy.com/docs/v1.0.x/configuration/#usage states that:

Individual config properties can be pure values or functions that receive the context object and return a value.

// config/deploy.js
module.export = function(deployTarget){
 var ENV = {
   redis: {
     revisionKey: function(context) {
       return context.deployment.tag;
     }
   }
 }
};

While the code says differently: https://github.com/ember-cli-deploy/ember-cli-deploy-plugin/blob/master/index.js#L68-L70

This PR amends the documentation to reflect the current behaviour.

PR Checklist

• Add tests
• Add documentation
• Prefix documentation-only commits with [DOC]