Add first page watermark like arXiv for PDFs

[ghseeli]

In particular, I want some text of the form "Built from commit ###### of github.com/ghseeli/grad-school-writings" so that users can reference specifically which source the pdf came from.

[ghseeli]

Some ideas about how to do this: https://tex.stackexchange.com/questions/7400/watermark-on-first-page-in-left-margin-like-arxiv