[DOC] owl: add generic file level documentation

src/index.ts

@@ -1,7 +1,15 @@
+/**
+ * This file is the main file packaged by rollup (see rollup.config.js).  From
+ * this file, we export all public owl elements.
+ *
+ * Note that dynamic values, such as a date or a commit hash are added by rollup
+ */
 export { Component } from "./component";
 export { EventBus } from "./event_bus";
 export { Observer } from "./observer";
 
+// we need to import manually the extra directives so they can register
+// themselves in QWeb, otherwise these files will not even be loaded.
 import "./qweb_directives";
 import "./qweb_extensions";
 export { QWeb } from "./qweb_core";