diff --git a/README.md b/README.md
index 1576973..2f3d057 100644
--- a/README.md
+++ b/README.md
@@ -5,6 +5,7 @@ Yubikey Book.
 
 1. https://git.hatter.ink/collect/yubikey-handbook
 1. https://github.com/drduh/YubiKey-Guide
+1. https://docs.yubico.com/yesdk/users-manual/yubikey-reference/transports/overview.html
 
 
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