housekeeping: Mention Events.XamEssentials package

[worldbeater]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

This PR mentions in README.md that ReactiveUI has a new package named ReactiveUI.Events.XamEssentials published.

What is the current behavior? (You can also link to an open issue here)

There is no info that we support Xamarin.Essentials.

What is the new behavior (if this is a feature change)?

There is info that we support Xamarin.Essentials! 🥇

What might this PR break?

Nothing.

[codecov]

Codecov Report

Merging #1889 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1889   +/-   ##
======================================
  Coverage    66.4%   66.4%           
======================================
  Files         110     110           
  Lines        4387    4387           
  Branches      579     579           
======================================
  Hits         2913    2913           
  Misses       1305    1305           
  Partials      169     169

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