[NO-TICKET] Update README section on how to add new dependency group

[Strech]

What does this PR do?

Adds new section in the readme and remove duplicated section about how to add new test group into your setup, step by step.

Motivation:

I was able to do it only after the call, it makes sense to share that knowledge

Change log entry
No.

Additional Notes:

Might need some wording advice

How to test the change?

Follow instructions, and if it works - everything is correct then.

[datadog-datadog-prod-us1]

Datadog Report

Branch report: update-development-guide-testing-section
Commit report: 0916b42
Test service: dd-trace-rb

✅ 0 Failed, 22416 Passed, 1477 Skipped, 5m 30.27s Total Time
❄️ 1 New Flaky

New Flaky Tests (1)

• sucker_punch instrumentation successful job instruments successful enqueuing - rspec - Last Failure

  Expand for error

   Detected invalid array contents due to unsynchronized modifications with concurrent users
   
   Failure/Error:
     traces.collect(&:spans).flatten.sort! do |a, b|
       if a.name == b.name
         if a.resource == b.resource
           if a.start_time == b.start_time
             a.end_time <=> b.end_time
           else
             a.start_time <=> b.start_time
   ...
  

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-22 12:02:34

Comparing candidate commit 0916b42 in PR branch update-development-guide-testing-section with baseline commit 97d8583 in branch master.

Found 1 performance improvements and 2 performance regressions! Performance is the same for 28 metrics, 2 unstable metrics.

scenario:profiler - stack collector

• 🟩 throughput [+184.874op/s; +186.903op/s] or [+6.732%; +6.805%]

scenario:tracing - 100 span trace - no writer

• 🟥 throughput [-18.293op/s; -17.384op/s] or [-5.266%; -5.005%]

scenario:tracing - Propagation - Datadog

• 🟥 throughput [-3235.130op/s; -3147.119op/s] or [-9.804%; -9.537%]

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.71%. Comparing base (97d8583) to head (0916b42).
Report is 22 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4307      +/-   ##
==========================================
- Coverage   97.72%   97.71%   -0.01%     
==========================================
  Files        1359     1359              
  Lines       82487    82487              
  Branches     4200     4200              
==========================================
- Hits        80609    80606       -3     
- Misses       1878     1881       +3     

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[ivoanjo]

Left a few comments!

[ivoanjo]

👍 LGTM