Create a new bookmark titled something like "Send to Omnifocus" or "✅", and paste this into the Location field. (Unfortunately for this purpose, but fortunately for less benevolent purposes, GFMD doesn't support javascript: links.)
javascript:q=location.href;if(document.getSelection){d='\n\n'+document.getSelection();}else{d='';};p=document.title;location.href='omnifocus:///add?name='+encodeURIComponent(p)+'¬e='+encodeURIComponent(q)+encodeURIComponent(d);
It will open the OmniFocus Quick Add dialog with the title of the link as the task name, and the URL and optionally any selected text as a Note.
Bookmarklets don't work on github.com. (STILL.) Test it out somewhere other than this page!