Add documentation

 # 1.3.4
 * Fix dart lint warnings
+* Add documentation
 
 # 1.3.3
 * Update Readme

@@ -16,6 +16,8 @@
 
 part of printing;
 
+/// Loads an image from a Flutter [ui.Image]
+/// into a [PdfImage] instance
 Future<PdfImage> pdfImageFromImage(
     {@required PdfDocument pdf, @required ui.Image image}) async {
   final ByteData bytes =
@@ -27,6 +29,8 @@ Future<PdfImage> pdfImageFromImage(
       height: image.height);
 }
 
+/// Loads an image from a Flutter [ImageProvider]
+/// into a [PdfImage] instance
 Future<PdfImage> pdfImageFromImageProvider(
     {@required PdfDocument pdf,
     @required ImageProvider image,

@@ -41,6 +41,9 @@ mixin Printing {
     }
   }
 
+  /// Prints a Pdf document to a local printer using the platform UI
+  /// the Pdf document is re-built in a [LayoutCallback] each time the
+  /// user changes a setting like the page format or orientation.
   static Future<void> layoutPdf(
       {@required LayoutCallback onLayout, String name = 'Document'}) async {
     _onLayout = onLayout;
@@ -49,6 +52,7 @@ mixin Printing {
     await _channel.invokeMethod<void>('printPdf', params);
   }
 
+  /// Prints a [PdfDocument] or a pdf stream to a local printer using the platform UI
   @deprecated
   static Future<void> printPdf({PdfDocument document, List<int> bytes}) async {
     assert(document != null || bytes != null);
@@ -59,6 +63,7 @@ mixin Printing {
             document != null ? document.save() : bytes);
   }
 
+  /// Displays a platform popup to share the Pdf document to another application
   static Future<void> sharePdf(
       {PdfDocument document, List<int> bytes, Rect bounds}) async {
     assert(document != null || bytes != null);

@@ -17,6 +17,7 @@
 part of printing;
 
 class PdfDoc extends Document {
+  /// Wrapper for a [Document] with zlib compression enabled by default
   PdfDoc({bool compress = true})
       : super(deflate: compress ? zlib.encode : null);
 }