chore(typeahead): add comment on typeahead-min-length

- Add comment on `typeahead-min-length` about valid values

Closes angular-ui#4129

Author: bsiddiqui

src/typeahead/docs/readme.md

@@ -42,7 +42,7 @@ The typeahead directives provide several attributes:
 
 * `typeahead-min-length` <i class="glyphicon glyphicon-eye-open"></i>
    _(Defaults: 1)_ :
-   Minimal no of characters that needs to be entered before typeahead kicks-in
+   Minimal no of characters that needs to be entered before typeahead kicks-in. Must be greater than or equal to 1.
 
 * `typeahead-no-results` <i class="glyphicon glyphicon-eye-open"></i>
    _(Defaults: angular.noop)_ :