Add readme to Controls package

[michael-hawker]

Describe the bug

We should add a readme to the Controls package like the WinUI team now that we have a dependency on them.

We should call out the same info about resources (required for things like the ColorPicker), but also that the Controls package is a meta package now and apps can be optimized by grabbing sub packages.

Hello michael-hawker, thank you for opening an issue with us!

I have automatically added a "needs triage" label to help get things started. Our team will analyze and investigate the issue, and escalate it to the relevant team if possible. Other community members may also look into the issue and provide feedback 🙌