[png-mng-misc] AGF Level 2 announce

[Alexey S. <ac...@gm...>]

I early gived draft of AGF preview ( http://acterhd.github.io/AGF-draft/ ).
Now, I extends format AGF by primitives. I present AG2 specification. AGF
Level 2 ( https://acterhd.github.io/AGF-draft/level-2.html ). Level 2 is
not version. All critical changes go to basic and extended version.