Improve README.md

[kondratevdev]

Hey! Thanks a lot for the project! This update makes the README friendlier and clearer for new users, so it is easier to quickly understand what subatomic does and why they might want it.

[samueljsb]

Hi, thanks for suggesting these changes 🙂

We'd like to keep all the documentation in one place, rather than have two places to maintain (we've noticed from experience that they often end up drifting out of sync when there are multiple), so most of the additions here we'd like to keep in just the docs.

The list of features is an interesting addition, though -- we might want to workshop the exact wording a bit, but we think they could be useful in the first page of the docs, above the "Quick Comparison" section, to provide a bit more context.