NOTE: I'm not a SEO guy. If this post used wrong wordings, sorry for that. And, I'm not a native English speaker, don't blame my English writing :P And, I don't know whether someone has shared something similar :)
I have several pet projects hosted on github. And, I observed that Google give pretty high ranking to github repository. If a github repository has a homepage, that homepage will get benefits as well.
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" :)