Add twitter badge to gh and website

[franklin-feingold]

Adding a BIDS twitter badge/reference to the:

• github readme
• RTD rendering (probably on the index)
• BIDS .io website

Please feel free to add other locations our twitter badge can be

Edit: #268 addresses point 1 and 2
bids-standard/bids-website#49 addresses point 3

[sappelhoff]

What badge did you have in mind? Or are you talking about a handle in text form like: @BIDSstandard

[franklin-feingold]

I was thinking of something like this

but a follow us badge. not a text handle but the visual figure

[KirstieJane]

You can make a badge in the README (and anywhere else!) using shields.io: https://shields.io/category/social

http://img.shields.io/twitter/follow/bidsstandard.svg?style=social

renders as

[franklin-feingold]

Yes! that's perfect thank you @KirstieJane !