Explain the ecmascript-runtime => promise dependency

packages/ecmascript-runtime/package.js

@@ -13,6 +13,9 @@ Npm.depends({
 
 Package.onUse(function(api) {
   api.use("modules");
+
+  // Regenerator, which we use to transpile ES2016 async/await, needs
+  // a promise implementation
   api.use("promise");
 
   api.mainModule("runtime.js");