Google would give github repo pretty high ranking. If a repo's README has a link to a page outside github, that page gets SEO bonus.edit: 20130308 If a repo's homepage has set to a web page, that page
would get SEO bonus from Google (I didn't carry out experiment on the
effect of the README yet). And, if the page links back to the github
repo, that's a plus.
Search google with the query "Github SEO effect" :)