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

Old repo link: https://github.com/3design/gbc/issues/20