Add example of how to set UID for user agents via settings.php to README

Problem/Motivation

It is mentioned in the settings form that the UIDs may differ across different environments and so are best set via settings.php file.

Only, an example of what to add would be useful, with a reference on the settings form /admin/config/workflow/minikanban/agents to this example in the README.

Steps to reproduce

Install and enable the minikanban agent module and go to the settings form /admin/config/workflow/minikanban/agents.

Proposed resolution

I have already tried to look up how to do this myself, if this is the correct way then this could possibly be added to the README.

$settings['minikanban_agent.settings']['main_user'] = '1';
$settings['minikanban_agent.settings']['fallback_approval_user'] = '1';
$settings['minikanban_agent.settings']['fallback_worker_user'] = '1';

After which the description in code should also be modified to mention that an example exists.

E.g. "This is the FICTIONAL user that will be assigned all the tasks and that you can assign epics to for generation. This syncs on UID which might be different per environment, so please use settings.php to set this for each. Example of this can be found in the README of the module."

Remaining tasks

Decide how best to solve this problem, modify the code, create an MR, review MR and merge if correct.

User interface changes

Some extra text will be added to the settings from.