Improve README

[alamb]

1. Add documentation on available features
2. Move some items to their own sections for easier discoverability

Rendered version here: https://github.com/alamb/sqlparser-rs/blob/alamb/improve_readme/README.md

[coveralls]

Pull Request Test Coverage Report for Build 3794961608

• 0 of 1(0.0%) changed or added relevant line in 1 file are covered.
• 268 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 85.974%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib.rs	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/lib.rs	268	10.02%

Totals
Change from base Build 3794845435:	-0.01%
Covered Lines:	13007
Relevant Lines:	15129

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💛 - Coveralls

[alamb]

I am working on improving the documentation of this crate and so I am going to merge this in prior to review.

I would love any comments on these changes and will be happy to make changes as a follow on PR

cc @tustvold and @AugustoFKL