Add readme to jetty-9.3 instrumentation

[jasonjkeller]

Adds a readme that describes how the Jetty instrumentation works with Jetty Continuations and the servlet async API.

[codecov-commenter]

Codecov Report

Merging #1539 (3de7598) into main (dbbbeb2) will decrease coverage by 0.03%.
Report is 22 commits behind head on main.
The diff coverage is 76.05%.

❗ Current head 3de7598 differs from pull request most recent head 6ddb672. Consider uploading reports for the commit 6ddb672 to get more accurate results

@@             Coverage Diff              @@
##               main    #1539      +/-   ##
============================================
- Coverage     70.60%   70.58%   -0.03%     
- Complexity     9747     9754       +7     
============================================
  Files           813      814       +1     
  Lines         39268    39300      +32     
  Branches       5963     5967       +4     
============================================
+ Hits          27727    27739      +12     
- Misses         8856     8870      +14     
- Partials       2685     2691       +6     

Files	Coverage Δ
.../com/newrelic/agent/attributes/AttributeNames.java	0.00% <ø> (ø)
...com/newrelic/agent/config/SecurityAgentConfig.java	85.00% <100.00%> (+0.78%)	⬆️
...ntation/context/InstrumentationContextManager.java	56.19% <ø> (-0.36%)	⬇️
...com/newrelic/agent/service/ServiceManagerImpl.java	77.61% <100.00%> (-0.37%)	⬇️
...elic/agent/service/analytics/SpanEventFactory.java	88.69% <100.00%> (+0.49%)	⬆️
...lic/agent/service/analytics/TracerToSpanEvent.java	98.01% <100.00%> (+0.01%)	⬆️
...relic/agent/service/module/JarCollectorInputs.java	100.00% <100.00%> (ø)
...in/java/com/newrelic/agent/utilization/CGroup.java	100.00% <100.00%> (ø)
.../agent/service/module/JarCollectorServiceImpl.java	80.55% <50.00%> (-2.78%)	⬇️
...ation/context/InstrumentationClassTransformer.java	65.00% <50.00%> (-0.79%)	⬇️
... and 3 more

... and 10 files with indirect coverage changes

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