What problem does this address?

The spacing-sizes-control component is missing a README.md file. As per #22891

What is your proposed solution?

Pinging original creator of this component @glendaviesnz. and other recent committers @t-hamano and @aaronrobertshaw if you have the bandwidth could you add a README.md file for this component. Thank you.

This README is a good example to follow.

Please let me know if there's anything I can do to help with the creation of the README.md file for this component.