Add archive notice to README

[dougbrn]

Added an archive notice indicating that the repository is no longer maintained and details about its migration to LSDB.

[github-actions]

Before [028664d]	After [9a68cd6]	Ratio	Benchmark (Parameter)
136M	136M	1	benchmarks.NestedFrameAddNested.peakmem_run
283±2ms	283±1ms	1	benchmarks.NestedFrameAddNested.time_run
143M	143M	1	benchmarks.NestedFrameQuery.peakmem_run
136M	136M	1	benchmarks.NestedFrameReduce.peakmem_run
292±2ms	291±1ms	0.99	benchmarks.NestedFrameReduce.time_run
877±10ms	862±5ms	0.98	benchmarks.NestedFrameQuery.time_run

Click here to view all benchmarks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.78%. Comparing base (028664d) to head (1b39ffe).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #96   +/-   ##
=======================================
  Coverage   95.78%   95.78%           
=======================================
  Files           9        9           
  Lines         261      261           
=======================================
  Hits          250      250           
  Misses         11       11           

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