docs(README): Update Onyx README

[ElijahKotyluk]

README:

• What is Onyx?
• Why use Onyx?
• Roadmap
• Contribution Guidelines

[codecov]

Codecov Report

Merging #10 into master will increase coverage by 0.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
+ Coverage   97.95%   98.23%   +0.27%     
==========================================
  Files          22       23       +1     
  Lines         147      170      +23     
  Branches       27       28       +1     
==========================================
+ Hits          144      167      +23     
  Misses          3        3

Impacted Files	Coverage Δ
packages/matchers/src/expect.ts	100% <0%> (ø)	⬆️
packages/matchers/src/toThrow.ts	100% <0%> (ø)	⬆️
packages/matchers/src/matchers.ts	100% <0%> (ø)

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