Add note about adding runner token to gdk.yml

What does this Merge Request do?

whilst working with the gdk this morning, i ran into an issue where gdk reconfigure was replacing my my runner's token with a placeholder. whilst working through this with help from @ashmckenzie, we thought it might be good to add this to the docs to signpost it a bit more prominently.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.