Add donation to shields.io to budget

[mattbk]

Since we seem to get getting a bunch of new projects from them (awareness, #1114), we should support them. https://gratipay.com/shields/

[chadwhitacre]

How much should we pay? What is the value we get from them?

• they host Gratipay badges for us
• they send us traffic
• we use some of their other badges on our own README

Looking through our budget I think they should be down towards the bottom. How about $1/wk?

[mattbk]

I am happy with that.