Update ProgressIndicator.md

[betrue-final-final]

Adding design documentation for Progress Indicator.

[msftclas]

Hi @betrue-final-final, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

It looks like you're a Microsoft contributor (Ben Truelove). If you're full-time, we DON'T require a Contribution License Agreement. If you are a vendor, please DO sign the electronic Contribution License Agreement. It will take 2 minutes and there's no faxing! https://cla.microsoft.com.

TTYL, MSBOT;

[mikewheaton]

Approved 👍

[wdo3650]

Approved