Add fix to README.md for MongoDB-CR authentication failure

[harenson]

Go to mongoDB console and delete your current user & set authSchema version to 3 instead of 5 , follow these commands in mongo console -

mongo
use admin
db.system.users.remove({})    <== removing all users
db.system.version.remove({}) <== removing current version 
db.system.version.insert({ "_id" : "authSchema", "currentVersion" : 3 })

Now restart the mongod and create new user then it should work fine.

Note: use remove commands in test db only, if in production use update.

Fix taken from: http://stackoverflow.com/a/31476552

[harenson]

Closed in inaka/sumo_db_mongo#3