README.mux: add a reference to the AnimDecoder API
README.mux: add a reference to the AnimDecoder API

this balances the AnimEncoder section

Change-Id: I205c8d0bd6104509e06737dcbf9a7651fd4bc6a3

PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp