IMAGING-341: Document the PNG specification version

Author: Glavo

src/main/java/org/apache/commons/imaging/formats/png/package-info.java

@@ -16,7 +16,14 @@
  */
 
 /**
- * The PNG image format.
+ * The PNG (Portable Network Graphics) image format.
+ * <p>
+ * The implementation is based on PNG specification version 1.2, and supports the following extensions:
+ * <ul>
+ *     <li><a href="http://ftp-osl.osuosl.org/pub/libpng/documents/pngext-1.5.0.html">Extensions to the PNG 1.2 Specification, Version 1.5.0</a></li>
+ * </ul>
+ *
+ * @see <a href="http://www.libpng.org/pub/png/spec/1.2/">Portable Network Graphics (PNG) Specification, version 1.2</a>.
  */
 package org.apache.commons.imaging.formats.png;