make finding out how to make a transparent image simpler

Imager.pm

@@ -4581,6 +4581,9 @@ text, measuring - L<Imager::Font/bounding_box>, L<Imager::Font::BBox>
 
 tiles, color - L<Imager::Filters/mosaic>
 
+transparent images - L<Imager::ImageTypes>,
+L<Imager::Cookbook/"Transparent PNG">
+
 =for stopwords unsharp
 
 unsharp mask - L<Imager::Filters/unsharpmask>

lib/Imager/Cookbook.pod

@@ -91,6 +91,25 @@ say TIFF to JPEG, you'll need multiple output files:
       or die $image->errstr;
   }
 
+=head2 Transparent PNG
+
+To save to a transparent PNG (or GIF or TIFF) you need to start with
+an image with transparency.
+
+To make a transparent image, create an image object with 2 or 4
+channels:
+
+  # RGB with alpha channel
+  my $rgba = Imager->new(xsize => $width, ysize => $height, channels => 4);
+
+  # Gray with alpha channel
+  my $graya = Imager->new(xsize => $width, ysize => $height, channels => 2);
+
+By default, the created image will be transparent.
+
+Otherwise, if you have an existing image file with transparency,
+simply read it, and the transparency will be preserved.
+
 =head1 IMAGE SYNTHESIS
 
 =head2 Creating an image

lib/Imager/ImageTypes.pod

@@ -102,15 +102,17 @@ stored.  Imager treats these as follows:
 
 =item *
 
-2 samples per color - gray scale image with alpha channel.
+2 samples per color - gray scale image with alpha channel, allowing
+transparency.
 
 =item *
 
 3 samples per color - RGB image.
 
 =item *
 
-4 samples per color - RGB image with alpha channel.
+4 samples per color - RGB image with alpha channel, allowing
+transparency.
 
 =back