Make readme links relative
Make readme links relative

Having absolute `href`s meant that the table of content behaved unexpectedly on `http://npmjs.com/package/react-flip-move`. They would navigate back to GitHub, rather than an element in the alreadty rendered document.

joshwcomeau/react-flip-move,joshwcomeau/react-flip-move,joshwcomeau/react-flip-move