Expose and document SlideIndicator contract

Author: DavidDWiser

lib/flutter_carousel_widget.dart

@@ -9,3 +9,4 @@ export 'package:flutter_carousel_widget/src/indicators/circular_slide_indicator.
 export 'package:flutter_carousel_widget/src/indicators/circular_static_indicator.dart';
 export 'package:flutter_carousel_widget/src/indicators/circular_wave_slide_indicator.dart';
 export 'package:flutter_carousel_widget/src/indicators/sequential_fill_indicator.dart';
+export 'package:flutter_carousel_widget/src/indicators/slide_indicator.dart';

lib/src/indicators/slide_indicator.dart

@@ -1,5 +1,18 @@
 import 'package:flutter/material.dart';
 
+/// Abstraction used as a contract for building a slide indicator widget.
+///
+/// See also:
+///
+///  * [CircularSlideIndicator]
+///  * [CircularStaticIndicator]
+///  * [CircularWaveSlideIndicator]
+///  * [SequentialFillIndicator]
 abstract class SlideIndicator {
+  /// Builder method returning the slide indicator widget.
+  ///
+  /// The method accepts the [currentPage] on which the carousel currently
+  /// resides, the [pageDelta] or the difference between the current page and
+  /// its resting position and [itemCount] which is the total number of items.
   Widget build(int currentPage, double pageDelta, int itemCount);
 }