README.md: add PNG/GIF

Only in brackets - do not advertise those much because the encoder/decoder
may be slow.

Author: MartinPulec

README.md

@@ -59,7 +59,8 @@ Features
 - Decoder can decompress JPEG codestreams that can be generated by encoder. If scan
   contains restart flags, decoder can use parallelism for fast decoding.
 -  _command-line_ tool with support for encoding/decoding **raw** images
-as well as **BMP**, **TGA**, **PNM/PAM** or **Y4M**
+as well as **BMP**, **TGA**, **PNM/PAM** or **Y4M** (**PNG** and **GIF** also
+available for convenience)
 
 Overview
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