fix links in the README.md
fix links in the README.md

The links on the README are pointing into nowhere. For example: https://github.com/rom-rb/rom/blob/master/rom-rb/rom-relation
rom-rb/rom,jeremyf/rom,endash/rom,kwando/rom,vrish88/rom,cored/rom,rom-rb/rom,denyago/rom,pvcarrera/rom,Snuff/rom,dcarral/rom,rom-rb/rom,dekz/rom,pdswan/rom,pxlpnk/rom