docs: guide on augmenting FetchOptions

[kricsleo]

resolve #103

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.82%. Comparing base (27996d3) to head (7e01339).
Report is 67 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #487       +/-   ##
===========================================
+ Coverage   56.86%   69.82%   +12.96%     
===========================================
  Files          16       17        +1     
  Lines         728      527      -201     
  Branches      113      138       +25     
===========================================
- Hits          414      368       -46     
+ Misses        303      148      -155     
  Partials       11       11               

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.