Update mongo.rst

1. Use db.createUser() instead of db.addUser() to add users to MongoDB. Deprecated since version 2.6
2. NodeBB causes mongo error during setup procedure. Change the role from "userAdmin" to "readWrite".
fixed #1584
closed #1437
category widget changes
closed #1586
closes #1577
Merge pull request #1583 from tomcheung789/patch-1

Update mongo.rst
closes #1476
closes #1590, closes #1589