Document how to run the ncm

[story645]

Create a checklist of what needs to be in place - social, permissions, agenda - before the new contributors meeting:

• where the social media cards/template are
• posting schedule
• links to agenda template
• who has hosting permissions
• how to update the times on the scientific-python calender
• etc

[story645]

closed by https://github.com/matplotlib/ProjectManagement/blob/master/ncm_admin.md