Stamp revisions as revisions

When papers are revised (e.g. [W17-0807](https://aclweb.org/anthology/papers/W/W17/W17-0807/)), we swap links around so that the paper identifier (W17-0807) now points to the revised document. This is easy to not notice.

It'd be good to include a stamp on the PDF (as arXiv does) to indicate that a revision is a revision (and possibly that an original has been revised).

I know there's a broader conversation on how to handle revisions. I'd like to put my tuppence in: We request that authors who cite arXiv cite the version number; it would be nice if we provided these ourselves. The first version should be v1, etc., with the paper ID also pointing to the latest version.