📝 Document Mutual TLS (mTLS) usage with example in README

[pboling]

• closes Example: RFC 8705 - OAuth MTLS #583

[coveralls]

Pull Request Test Coverage Report for Build 17356057781

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 17355954358:	0.0%
Covered Lines:	519
Relevant Lines:	519

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💛 - Coveralls

[github-actions]

Package	Line Rate	Branch Rate	Health
oauth2	100%	100%	✔
Summary	100% (519 / 519)	100% (174 / 174)	✔

Minimum allowed line rate is 100%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (310ccce) to head (5c08737).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #665   +/-   ##
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