use admin
switched to db admin
rs0:PRIMARY> db.createUser({user: "RootAdmin", pwd: "gauridara", roles: [ { role: "root", db: "admin" } ]});
2017-01-21T18:19:09.814+0000 E QUERY    [main] Error: couldn't add user: not authorized on admin to execute comm
and { createUser: "RootAdmin", pwd: "xxx", roles: [ { role: "root", db: "admin" } ], digestPassword: false, writ
eConcern: { w: "majority", wtimeout: 300000.0 } } :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
DB.prototype.createUser@src/mongo/shell/db.js:1290:15
@(shell):1:1