Add Known limitations in README

[clairewu0218]

Pull Request Template

Description

Add the list of unsupported controls to README

Checklist

• [X] I have performed a self-review of my own code

[github-actions[bot]]

Package	Line Rate	Branch Rate	Complexity	Health
Microsoft.PowerApps.TestEngine	94%	91%	810	✔
Summary	94% (2155 / 2282)	91% (509 / 560)	810	✔

Minimum allowed line rate is 50%