Support externalDocs annotation comment

[ikrisliu]

Describe the PR
Support externalDocs annotation comment.

Relation issue
#470

[ubogdan]

Can you add unit tests to cover the newly added code?

Readme should also be updated so new users will discover this feature easily

[ikrisliu]

Can you add unit tests to cover the newly added code?

Readme should also be updated so new users will discover this feature easily

Done

[codecov]

Codecov Report

Base: 85.14% // Head: 85.18% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (e835e89) compared to base (aa3e8d5).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head e835e89 differs from pull request most recent head 3d9890b. Consider uploading reports for the commit 3d9890b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1468      +/-   ##
==========================================
+ Coverage   85.14%   85.18%   +0.03%     
==========================================
  Files          19       19              
  Lines        3616     3624       +8     
==========================================
+ Hits         3079     3087       +8     
  Misses        459      459              
  Partials       78       78              

Impacted Files	Coverage Δ
parser.go	94.50% <100.00%> (+0.05%)	⬆️

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[ubogdan]

@ikrisliu Thanks for your contribution