Update readme.md

feat(typeahead): Clarify documentation on the type of event that will cause should-select to be called

Author: christor

Diff for: src/typeahead/docs/readme.md

@@ -80,7 +80,7 @@ This directive works with promises, meaning you can retrieve matches using the `
 * `typeahead-should-select($event)`
   <small class="badge">$</small>
   _(Default: `null`)_ -
-  A callback executed when an event occurs that might trigger a selection. Selection will only occur if this function returns true.
+  A callback executed when a `keyup` event that might trigger a selection occurs. Selection will only occur if this function returns true.
 
 * `typeahead-on-select($item, $model, $label, $event)`
   <small class="badge">$</small>