Update README.md

[neonsigh]

Notation for which stretch goals are currently included in build and added section detailing additional features beyond mvp features and stretch goals

Checklist:

• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have checked my code and corrected any misspellings
• [x] I will put an appropriate label on my pull request

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