Better documentation for putc callback

[charlesnicholson]

The readme doesn't give an example of the actual npf_putc callback, just how to call nanoprintf with a callback.

Add an actual callback, with use of the context pointer, to the readme to make it clearer.

[charlesnicholson]

Additionally, give an example of a variadic user-defined print function that forwards to npf_vpprintf, to show how a per-byte callback setup might work.

[charlesnicholson]

follow-up from #240