Update README.md

README.md

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 U2F Zero
 ========
 
-U2F Zero is an open source U2F token for 2 factor authentication.  It is implemented securely.  It works with Google accounts, Github, Duo, and anything else supporting U2F.  It can be used with 15 services at a time.
+U2F Zero is an open source U2F token for 2 factor authentication.  It is implemented securely.  It works with Google accounts, Github, Duo, and anything else supporting U2F.  The latest version uses key derivation and has no limit on registrations.
 
 ![](http://i.imgur.com/dQpo9wC.jpg "The door keys are not used")