Use link markup instead of rdoc-ref for URLs in README

You will know your audience better than me, but the links in the README that use `rdoc-ref` to markup the links do not work in Github. If you expect most people to view the README through an RDoc generator/viewer of some sort than the current optimization for RDoc is likely preferable. If the Github use-case is valid one, then consider this PR.

This PR switches those links to use `link` to markup the links instead.

According to the [RDoc link docs](http://docs.seattlerb.org/rdoc/RDoc/Markup.html#class-RDoc::Markup-label-Links):
 > Use rdoc-ref: instead of link: to link to files generated by RDoc as the link target may be different across RDoc generators.

Again, you will probably know better than I would if there is likely to be variation in the target of those links across different RDoc generators. I'm skeptical, but I know relatively little about RDoc.

Since using `rdoc-ref` seems pretty reasonable, I've also opened [an issue with Github](https://github.com/github/markup/issues/420).