Add branding to the GitHub org

[jacobtomlinson]

It would be nice to update the logo on https://github.com/pangeo-data to use the branding.

I don't have appropriate permissions to change it. Could someone do it or give me the perms?

[rabernat]

Thanks for doing this!

However, I have to admit that I really dislike the color scheme on that particular permutation of the logo. (I made the same comment here: #78 (comment))
The dueling green blue gradients look super clashy to me.

Could we standardize around this one instead?

Or perhaps do another design iteration?

[jacobtomlinson]

Happy to go with the white background!