docs(typeahead): clarify subset of select's syntax used

Closes #715
Closes #506

src/typeahead/docs/readme.md

@@ -1,8 +1,12 @@
-Typeahead is a AngularJS version of [Twitter Bootstrap typeahead plugin](http://twitter.github.com/bootstrap/javascript.html#typeahead)
-
+Typeahead is a AngularJS version of [Twitter Bootstrap typeahead plugin](http://twitter.github.com/bootstrap/javascript.html#typeahead).
 This directive can be used to quickly create elegant typeheads with any form text input.
 
-It is very well integrated into the AngularJS as:
+It is very well integrated into the AngularJS as it uses subset of the
+[select directive](http://docs.angularjs.org/api/ng.directive:select) syntax, which is very flexible. Supported expressions:
+
+* _label_ for _value_ in _sourceArray_
+* _select_ as _label_ for _value_ in _sourceArray_
+
+The `sourceArray` expression can use a special `$viewValue` variable that corresponds to a value entered inside input by a user.
 
-* it uses the same, flexible syntax as the [select directive](http://docs.angularjs.org/api/ng.directive:select)
-* works with promises and it means that you can retrieve matches using the `$http` service with minimal effort
+Also this directive works with promises and it means that you can retrieve matches using the `$http` service with minimal effort.