Add AmyJune Hineline (volkswagenchick) as a provisional mentoring coordinator

The Drupal Core Mentoring Coordinators are delighted to invite AmyJune Hineline (volkswagenchick) to the list of Provisional Mentoring Coordinators, listed in MAINTAINERS.txt

We have discussed the responsibilities of the role and, indeed, agreed that a primary task for those joining as provisional members in this update will be to better document the processes we follow.