readme: Add instructions for making changes to the standard library
readme: Add instructions for making changes to the standard library

Closes #2324

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