Add information about filling in fields (or not) for a test atlas

[adamstone7]

Add a point in Step 5 of the tutorial explaining that while one's previous password can be used from Step 3.3, the username section should remain blank in cases where a test atlas is being created for the first time.

[adamstone7]

This may also be a bug in the basic atlas creation steps outlined in the tutorial. If one does enter a username in Step 5, the following error message appears when trying to update or run the newly-created atlas:

bash: ../nunaliit_2.2.9-SNAPSHOT_2020-08-17_c937f0e/bin/nunaliit: No such file or directory