Mention updating Team Page with Trainee Maintainer Status

Why is this change being made?

When following the documentation to become a trainee maintainer, I noticed there wasn't any mention of adding the information to the team.yml page. Which I believe is how Danger determines if you are a Trainee Maintainer or not when picking a reviewer!

I copied the verbiage from the bottom of the Trainee Frontend Maintainer Issue Template. I wasn't sure if wanted to go into detail of how to update your team.yml information here or not.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the change relates to any part of the project other than updates to content and/or data files (e.g. team.yml) please make sure to ping @gl-static-site-editor in a comment for a review and merge.
  • If the DRI for the page/s being updated isn't immediate clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies

For help with failing pipelines reach out in #mr-buddies in Slack