clarify readme : what do contributors need to know vs. what do admins/devs

[christinerogers]

Hi derek, Could you please update this so that at teh top it contains a non-technical section saying :

How to Contribute content

• GEt added as a collaborator on this repo by contacting ...
• Add an Event / News Item - here's how, and include a LINK to an example card.
• Add a person to the Network