Add instructions for finetuning owsm

[pyf98]

What?

This PR adds a step-by-step guide on how to fine-tune existing OWSM models on a specific dataset.

[codecov]

Codecov Report

Merging #5539 (241ec48) into master (a2a7d55) will increase coverage by 4.51%.
Report is 147 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5539      +/-   ##
==========================================
+ Coverage   70.68%   75.19%   +4.51%     
==========================================
  Files         711      711              
  Lines       65757    65757              
==========================================
+ Hits        46479    49449    +2970     
+ Misses      19278    16308    -2970     

Flag	Coverage Δ
test_configuration_espnet2	∅ <ø> (∅)
test_integration_espnet1	63.00% <ø> (ø)
test_integration_espnet2	48.61% <ø> (?)
test_python_espnet1	19.28% <ø> (ø)
test_python_espnet2	51.39% <ø> (+0.03%)	⬆️
test_utils	22.19% <ø> (ø)

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[sw005320]

Thanks, @pyf98!
I see how it is complicated.
@juice500ml and @simpleoier, it is a good example of making it work with a simple espnet