Remove duplicate asset definition

Thanks @csillag
Update README.rst

Add waffle.io badge.
Update README.rst

Should probably use the right link.
Update README.rst

Wow. Could I be more oblivious?
Update README.rst

Add info about the waffle badge and contributor prompt text.
Merge branch 'develop' of https://github.com/hypothesis/h into 514-simple-tagging-4
Tagging cosmetics

 - Input field (for new	tag) has border
 - Widget has no border
 - Input field retains placeholder (but	it is now shorter)