Merge pull request #310 from dirkjanm/master

Add notes about KDF compatibility (solves #307)

README.md

@@ -1292,7 +1292,7 @@ General key info..: [none]
 Use the [YubiKey Manager](https://developers.yubico.com/yubikey-manager) application (note, this is not the similarly named older YubiKey NEO Manager) to enable CCID functionality.
 
 ## Enable KDF
-Key Derived Function (KDF) enables YubiKey to store the hash of PIN, preventing the PIN from being passed as plain text.
+Key Derived Function (KDF) enables YubiKey to store the hash of PIN, preventing the PIN from being passed as plain text. Note that this requires a relatively new version of GnuPG to work, and may not be compatible with other GPG clients (notably mobile clients). These incompatible clients will be unable to use the YubiKey GPG functions as the PIN will always be rejected. If you are not sure you will only be using your YubiKey on supported platforms, it may be better to skip this step.
 
 ```console
 gpg/card> admin