added documentation for class and ID filters

README.md

@@ -16,3 +16,30 @@ This plugin will not work in ACF 4 for 2 reasons:
 * The JavaScript used in this plugin is specific to ACF 5
 
 This plugin also serves as a simple example of how to extend the functionality of fields in ACF 5.
+
+### Add Counter Filter
+By defaults, the counter is added to all text and textarea fields that have a max length. If you would like
+to only add the counter on specific fields you can filter them by either the ACF Wrapper Class or the ACF
+Wrapper ID of the field. You can allow multiple classes or ids. If classes or ID values are present then only
+fields that have one of the classes or ids will include a counter.
+
+**Filter by Class**
+```
+add_filter('acf-input-counter/classes', 'my_input_counter_filter');
+function my_input_counter_filter($classes=array()) {
+  // add 1 or more classes to the array
+	$classes[] = 'this-is-a-class';
+	return $classes;
+}
+```
+fields that have one of the classes or ids will include a counter.
+
+**Filter by ID**
+```
+add_filter('acf-input-counter/ids', 'my_input_counter_filter');
+function my_input_counter_filter($ids=array()) {
+  // add 1 or more classes to the array
+	$ids[] = 'this-is-an-ID';
+	return $ids;
+}
+```