Improve the "About Us" page

[tdooner]

As a Brigade leader, I want to know general background information about the brigade program so that I tell my members the story of the movement and how my brigade fits in.

Better information about what Brigades are, for new members

Should explain that it’s not just coders, but there are also other ‘types’ of contributors
Should explain mission
What’s the NAC? Who’s on the NAC?
How’s the Network structured?
Who’s the team?
How many Brigades are there?
Roughly how many people participate?

Todo list:

• Add a section describing the governance of the network - NAC and CfA (#478)
• Include the actual number of brigades that exist
• Add a section describing Code for All and linking to that
• Add a definition of what a Brigade actually is (US only? International?)
• Add some history of the Brigade program

[tdooner]

I think we're pretty happy with the "About Us" page now!