Update README to include more context and plugin onboarding steps.

[mch2]

Description

Update README to include more context on the build system and plugin onboarding steps.

Issues Resolved

Related #73

Check List

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[dblock]

@setiah @CEHENKLE This is very much design doc turned into documentation and does a great job at explaining what we have in the best place possible, next to the code, therefore I don't see why we need another doc checked into the repo that duplicates the code and its documentation

[dblock]

Added some more specific asks from looking at #315

[peternied]

Just started to onboard everyone to the new process, could we get this PR refreshed so we can point them to this doc?

[codecov-commenter]

Codecov Report

Merging #309 (fdd0e65) into main (0ac7e2b) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #309      +/-   ##
==========================================
- Coverage   69.65%   69.62%   -0.03%     
==========================================
  Files          59       59              
  Lines        1575     1577       +2     
==========================================
+ Hits         1097     1098       +1     
- Misses        478      479       +1     

Impacted Files	Coverage Δ
bundle-workflow/src/manifests/test_manifest.py	100.00% <0.00%> (ø)
...w/src/test_workflow/integ_test/integ_test_suite.py	0.00% <0.00%> (ø)

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[mch2]

Sorry for the lag here - updated.

[dblock]

Definitely fix merge conflict leftovers.

[dblock]

I don't love these steps. We want components to check that they can run ./bundle-workflow/build.sh manifests/1.1.0/opensearch-1.1.0 --snapshot end-to-end successfully. They can iterate with --component name to debug.

[mch2]

I can add a step for this explicitly, is there a step here you think can be removed?

[mch2]

Actually I'd rather merge this as is and have that updated when we merge this with #528