Add some documentation about the integration tests

[Grotax]

I wrote down some documentation on how to run the integration tests to make it a bit easier for new people.

Also added the missing kill command to the workflow.

[codecov-commenter]

Codecov Report

Merging #1882 (b521f72) into master (fca05d5) will decrease coverage by 0.17%. The diff coverage is 87.67%.

@@             Coverage Diff              @@
##             master    #1882      +/-   ##
============================================
- Coverage     91.62%   91.45%   -0.18%     
- Complexity      778      792      +14     
============================================
  Files            65       65              
  Lines          2724     2775      +51     
============================================
+ Hits           2496     2538      +42     
- Misses          228      237       +9     

Impacted Files	Coverage Δ
lib/Cron/UpdaterJob.php	0.00% <0.00%> (ø)
lib/Db/ItemMapperV2.php	98.51% <69.23%> (-1.49%)	:arrow_down:
lib/Service/FeedServiceV2.php	97.80% <81.81%> (-2.20%)	:arrow_down:
lib/Fetcher/FeedFetcher.php	80.62% <95.00%> (+0.89%)	:arrow_up:
lib/Controller/FeedController.php	100.00% <100.00%> (ø)
lib/Db/Item.php	100.00% <100.00%> (ø)
lib/Db/NewsMapperV2.php	100.00% <100.00%> (ø)
lib/Utility/OPMLExporter.php	100.00% <100.00%> (ø)
lib/Command/ExploreGenerator.php	100.00% <0.00%> (ø)
lib/Explore/RecommendedSites.php	0.00% <0.00%> (ø)
... and 7 more

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