[chore] [cmd/builder] update README.md

[jackgopack4]

Fix open-telemetry/opentelemetry-collector-releases#911

Description

Fixes documentation issue where ocb does not respect a default config file (needs to be specified via --config

Link to tracking issue

Fixes #911

Testing

None, doc only

Documentation

README.md

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.44%. Comparing base (3068122) to head (4461e90).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12821   +/-   ##
=======================================
  Coverage   91.44%   91.44%           
=======================================
  Files         487      487           
  Lines       26848    26848           
=======================================
  Hits        24551    24551           
  Misses       1814     1814           
  Partials      483      483           

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