Update "set admin user" script instructions

[Jérôme Mouneyrac]

The instructions to set up the admin user (https://docs.rowy.io/how-to/roles?set-user-roles-tabs=admin-sdk#set-user-roles-tabs) are a little time consuming as they mix import and require syntax. It makes setting up this mini project difficult.

As it is just a simple node script, not requiring modules, then the developer should just edit some constant in set-user-roles.js and then run:

yarn add firebase-admin
node ./set-user-roles.js

Currently the script produce errors with modules not supported, require not working, export not usable - all depending on how you decide to set your project.

PS: I am writing this issue to match a PR I am going to push.