Add link to releases page in readme

[dstillman]

I changed the QuickLook link on the plugins page to point to the front of the repo, since there's a lot of info that people won't see on the releases page. It'd be good to link "Releases" in the README to the releases page (and I'd suggest generally doing that instead of linking straight to GitHub releases pages, which can't provide any context).

[dstillman]

And the README should mention zoteroquicklook.zoteroplugin, not zoteroquicklook.xpi

https://forums.zotero.org/discussion/comment/359015/#Comment_359015