Create file UserDoc.md and add an entry about resetting password.

UserDoc.md

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+Resetting password
+-------------------
+
+We can reset a user's password by directly modifying the MySQL database table called `users_table`.  More specifically, we delete that user's information from `users_table` so that the user could sign up again.  Suppose the user's student number is 201131129138.
+
+To do so, LRR administrator logs in to MySQL using the following command:  `mysql -u username -p`.  Type the correct password to access the MySQL database.
+
+After that, issue the following commands in the mysql prompt.
+
+- `use lrr;`
+
+- `delete from users_table where Student_ID="201131129138";`
+
+The first one uses a database called lrr in MySQL.  The second one deletes a record from `users_table` where the student number is 201131129138.
+