readme: expand Developing Zig section into Contributing section
readme: expand Developing Zig section into Contributing section

raulgrell/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig