Document user creation for store management

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## Creating a user

BTCPayserver has two levels of user management: server-wide users and store

users. The latter depends on the former.

When adding a user, you'll first want to head over to `Server settings` in the

left menu and add a new user there. Leave the password field empty so that a

password reset URL will be sent to the new user's email address. By default, new

users are created without full server admin capabilities, which is usually what

we want.

Only if you need the new user to have full admin access, edit the user in the

list and set the "admin" flag there. Note that this is not necessary for

handling operations on the cryptocurrency store.

Once the server-wide user is created, head over to `Settings` in the left menu

and then to the tab `Users`. There, enter the email address of the system-wide

user you just created, select `Owner` and click on `Add User`. The only other

role that's present is `Guest`, which does not let one change store settings.

## Upgrades

Upgrades work by updating all container images and restarting the