Create passwd.md

Added a few exemples, and basic description of passwd

common/passwd.md

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+#passwd
+
+> passwd is a tool on most Unix and Unix-like operating systems used to change a user's password.
+
+* Change your password. ( You may need to use **sudo** )
+
+`passwd {{options}} {{new password}}`
+
+* Change the password of the specified user.
+
+`passwd {{username}} {{new password}}`
+
+* Make the password of the account blank ( It will set the named account passwordless. )
+
+`passwd -d`