add info about argon2 support

radicale/auth/htpasswd.py

@@ -46,6 +46,9 @@ out-of-the-box:
 When bcrypt is installed:
     - BCRYPT     (htpasswd -B ...) -- Requires htpasswd 2.4.x
 
+When argon2 is installed:
+    - ARGON2     (python -c 'from passlib.hash import argon2; print(argon2.using(type="ID").hash("password"))'
+
 """
 
 import functools