[Documentation] Create steps for adding users to GitHub Organization

[kjaymiller]

It is often that users will need to be added to the GitHub organization and necessary teams. It would be good to outline the steps to complete this tasks. This will mostly be linked to by other documentation.

To Complete

• Create a markdown document in the repo in a new folder called processes called joining the GitHub org.
• Detail the following
  • who is able to add to the org
  • the steps to add a member to the org