housekeeping: Update the Documentation and Code Snippets in README

[worldbeater]

What kind of change does this PR introduce?

This PR updates README.md with additional notes regarding up-to-date ReactiveUI.Validation features. This PR documents the changes introduced in #119 and #118, including the API changes and new ValidationRule overloads.

What is the current behavior?

Currently, the docs in README.md are a bit outdated.

What is the new behavior?

Now, the docs in README.md should be up-to-date.

What might this PR break?

Nothing. However, we may need to publish a new release for our NuGet to match the documentation.

[codecov]

Codecov Report

Merging #121 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #121   +/-   ##
=======================================
  Coverage   60.33%   60.33%           
=======================================
  Files          15       15           
  Lines         837      837           
=======================================
  Hits          505      505           
  Misses        332      332           

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