[AArnott]

Fixes #3799

NuGet.org flags both packages as missing a README. This adds a short, package-focused PackageReadme.md (what the package does, how to install it, how to configure it, and links to docs and release notes) and packs it as README.md in both StyleCop.Analyzers and StyleCop.Analyzers.Unstable via <readme>README.md</readme> in the two nuspecs.

I wrote a separate file rather than packing the repo's README.md, because that one uses relative links and badge images that don't render on NuGet.org, and its install section is about VS2015.

This is a packaging-only change, with no analyzer code or perf impact. minClientVersion stays 2.7. Like the existing <license> element, <readme> only needs a newer NuGet at pack time, and older clients ignore it.

Verification: the nuget-packages artifact from CI run 37950293681 has README.md at the root of both StyleCop.Analyzers.1.2.0-beta.862.g3cdb3c922d.nupkg and StyleCop.Analyzers.Unstable.1.2.0-g3cdb3c922d.nupkg, and both nuspecs contain <readme>README.md</readme>.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.54%. Comparing base (868775d) to head (6dd13ec).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4204   +/-   ##
=======================================
  Coverage   97.54%   97.54%           
=======================================
  Files        1277     1277           
  Lines      134157   134157           
  Branches     4291     4291           
=======================================
+ Hits       130863   130865    +2     
+ Misses       2152     2151    -1     
+ Partials     1142     1141    -1     

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