[query-engine] Add back RecordSet README deleted with PoC

[CodeBlanch]

Changes

• Adds back the README which was deleted with the PoC folder

Details

The main reason for this is it explains the current design for summary data which was discussed today on SIG call with @drewrelmas @albertlockett @lquerel and @jmacd.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.09%. Comparing base (3bfb8a2) to head (2c6746a).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1738      +/-   ##
==========================================
+ Coverage   83.96%   84.09%   +0.12%     
==========================================
  Files         467      470       +3     
  Lines      135720   136558     +838     
==========================================
+ Hits       113960   114834     +874     
+ Misses      21226    21190      -36     
  Partials      534      534              

Components	Coverage Δ
otap-dataflow	85.38% <ø> (+0.18%)	⬆️
query_abstraction	80.61% <ø> (ø)
query_engine	90.39% <ø> (ø)
syslog_cef_receivers	∅ <ø> (∅)
otel-arrow-go	53.50% <ø> (ø)

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