chore(docs): clarify a few things about local testing in the readme

[emmettbutler]

These changes make a few small improvements to the README that would have helped me when I was onboarding.

Checklist

• Change(s) are motivated and described in the PR description.
• Testing strategy is described if automated tests are not included in the PR.
• Risk is outlined (performance impact, potential for breakage, maintainability, etc).
• Change is maintainable (easy to change, telemetry, documentation).
• Library release note guidelines are followed.
• Documentation is included (in-code, generated user docs, public corp docs).
• Author is aware of the performance implications of this PR as reported in the benchmarks PR comment.

Reviewer Checklist

• Title is accurate.
• No unnecessary changes are introduced.
• Description motivates each change.
• Avoids breaking API changes unless absolutely necessary.
• Testing strategy adequately addresses listed risk(s).
• Change is maintainable (easy to change, telemetry, documentation).
• Release note makes sense to a user of the library.
• Reviewer is aware of, and discussed the performance implications of this PR as reported in the benchmarks PR comment.

[Yun-Kim]

Actually, this might clarify the fact the testrunner image is internal to this library and does not need to be installed from an external source.

[pr-commenter]

Benchmarks

Comparing candidate commit 7657b91 in PR branch emmett.butler/riot-readme with baseline commit 4f88161 in branch 1.x.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 56 cases.

[codecov-commenter]

Codecov Report

Merging #5144 (750db95) into 1.x (bd20c55) will decrease coverage by 0.01%.
The diff coverage is 86.09%.

@@            Coverage Diff             @@
##              1.x    #5144      +/-   ##
==========================================
- Coverage   74.63%   74.63%   -0.01%     
==========================================
  Files         848      848              
  Lines       65636    65705      +69     
==========================================
+ Hits        48988    49037      +49     
- Misses      16648    16668      +20     

Impacted Files	Coverage Δ
ddtrace/contrib/asgi/middleware.py	86.30% <77.77%> (-8.70%)	⬇️
ddtrace/contrib/django/patch.py	83.23% <94.11%> (+0.10%)	⬆️
ddtrace/contrib/django/utils.py	92.34% <100.00%> (+0.26%)	⬆️
tests/contrib/django/test_django_snapshots.py	100.00% <100.00%> (ø)
ddtrace/internal/remoteconfig/worker.py	92.30% <0.00%> (-3.85%)	⬇️
ddtrace/contrib/cassandra/session.py	86.09% <0.00%> (-1.07%)	⬇️
ddtrace/appsec/ddwaf/ddwaf_types.py	87.91% <0.00%> (-0.55%)	⬇️
tests/utils.py	90.18% <0.00%> (-0.24%)	⬇️

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