doc: update readme to use GetCredential first

[JacksonTian]

You need to complete

• unit tests and/or feature tests
• documentation is changed or added

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (971916d) 91.68% compared to head (35aeee4) 91.68%.

❗ Current head 35aeee4 differs from pull request most recent head 1b00502. Consider uploading reports for the commit 1b00502 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #56   +/-   ##
=======================================
  Coverage   91.68%   91.68%           
=======================================
  Files          20       20           
  Lines        1214     1214           
=======================================
  Hits         1113     1113           
  Misses         82       82           
  Partials       19       19           

Flag	Coverage Δ
credentials-go	91.68% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.