housekeeping: Mention the New Android Packages

[worldbeater]

What kind of change does this PR introduce?

This PR updates our README.md with additional documentation related to AndroidX and AndroidSupport.

What is the current behavior?

Currently, there is no info in README.md describing the features introduced in #134

What is the new behavior?

Now, the changes and new packages are documented. Ideally, a new 1.8 release should be published after merging this PR.

What might this PR break?

Those folks who are using ReactiveUI.Validation on MonoAndroid90 will have to install one of the new packages:

• ReactiveUI.Validation.AndroidSupport
• ReactiveUI.Validation.AndroidX

Closes #129

[codecov]

Codecov Report

Merging #136 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #136   +/-   ##
=======================================
  Coverage   61.57%   61.57%           
=======================================
  Files          16       16           
  Lines         877      877           
=======================================
  Hits          540      540           
  Misses        337      337           

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