Update todo webapp tutorial docs and change todo/import/Todo_Web_App_With_Twilio/ clusterName in config.json

[danielhelfand]

The todo webapp tutorial does not specify that you need to create a mongdb-atlas cluster when you import the application in the documentation and also makes the assumption that the user has named the cluster StitchDemoCluster as shown here.

I would suggest adding some information in the tutorial stating that the user should set up a cluster similar to what was done with the dashboard tutorial. I would also suggest changing the default clusterName in config.json to Cluster0 as this is the default name that M0 clusters are created with if the name is not changed.