Add migration guide

[martincostello]

Add a migration guide for moving from xunit v2 to v3.

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README.md:147

• The description should end with a period.

[repo]: https://github.com/martincostello/xunit-loggingE "This project on GitHub.com"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.53%. Comparing base (e39e9bf) to head (bdc9165).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #805   +/-   ##
=======================================
  Coverage   98.53%   98.53%           
=======================================
  Files          15       15           
  Lines         273      273           
  Branches       36       36           
=======================================
  Hits          269      269           
  Misses          2        2           
  Partials        2        2           

Flag	Coverage Δ
linux	97.80% <ø> (-0.74%)	⬇️
macos	97.80% <ø> (ø)
windows	98.53% <ø> (ø)

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