Update README to streamline core focus of the FAST project

[janechu]

Pull Request

📖 Description

This pull request simplifies the root level README in accordance with a re-alignment on core FAST goals.

We will be re-focusing the project around the @microsoft/fast-element package and a number of existing packages will have RFCs in the future for discussion around deprecation strategy.

✅ Checklist

General

• [ ] I have included a change request file using $ yarn change
• [ ] I have added tests for my changes.
• [ ] I have tested my changes.
• [x] I have updated the project documentation to reflect my changes.
• [x] I have read the CONTRIBUTING documentation and followed the standards for this project.