fix link to contributing page in readme
fix link to contributing page in readme

sampsyo/beets,jackwilsdon/beets,jackwilsdon/beets,sampsyo/beets,ibmibmibm/beets,ibmibmibm/beets,ibmibmibm/beets,sampsyo/beets,shamangeorge/beets,shamangeorge/beets,jackwilsdon/beets,jackwilsdon/beets,beetbox/beets,ibmibmibm/beets,beetbox/beets,shamangeorge/beets,sampsyo/beets,beetbox/beets,beetbox/beets,shamangeorge/beets