chore(readme): added nightly build section

[GitToTheHub]

Added section on installing nightly builds of Cordova Android for testing purposes, with warnings about stability and production use.

Platforms affected

Motivation and Context

Description

Testing

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.79%. Comparing base (d7afba0) to head (e831d71).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1873   +/-   ##
=======================================
  Coverage   60.79%   60.79%           
=======================================
  Files          23       23           
  Lines        4811     4811           
=======================================
  Hits         2925     2925           
  Misses       1886     1886           

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