Document the colors supported by the custom Shield

[olivierlacan]

We should list the colors supported since it appears that grey and gray don't work: http://img.shields.io/cheez/whiz.png?color=grey
http://img.shields.io/cheez/whiz.png?color=gray

That's one more point in the camp of #61 by the way.

[chadwhitacre]

What should the colors be, @olivierlacan? The new API server supports more than the old did:

https://github.com/gittip/img.shields.io/blob/ee348f618212883785591c3b07bc5c2cdf1c7375/README.md#valid-colours

[nathany]

@olivierlacan Can you commit to coming up with a style guide for the colors, gradients, fonts, dimensions, etc. for badge generation?

[nathany]

Also see this issue with regards to font selection with SVG. badges/gh-badges#14

[olivierlacan]

I think the colors are well documented on http://shields.io/

[scottschreckengaust]

I found that HEX values were case-sensitive...