Fix typos, add logo, add downloads badge to README

[worldbeater]

What kind of change does this PR introduce?

README.md file update.

What is the current behavior?

Currently there is a typo in the example, so mostly this PR is aimed to fix the following line:

// The property will create a new context each time it is accessed.
// This could cause issues if someone copy-pastes this line.
public ValidationContext ValidationContext => new ValidationContext();

What is the new behavior?

Fixed the typo, added shields downloads badge, added logo, added contributing section.

What might this PR break?

Nothing.

[glennawatson]

I will fix the build in a couple hours

[codecov]

Codecov Report

Merging #22 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   34.98%   34.98%           
=======================================
  Files          16       16           
  Lines         706      706           
  Branches       82       82           
=======================================
  Hits          247      247           
  Misses        451      451           
  Partials        8        8

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