Add gitignore

[thodson-usgs]

My collaborators had a tendency to accidentally commit PDF, which made it harder to incorporate their pull requests. Consider adding PDF to gitignore.

[sanjib-sen]

Good observation. Thanks. Although, I am not sure what will happen when a person not so good with gitignore suddenly finds that their PDFs are not updating after git sync! (Considering many people also commit their PDF's).

[thodson-usgs]

I'm okay making it (slightly) harder to commit a pdf, but that's up to you. Then perhaps ignore everything in PDF except the pdf's with

PDF/*
!PDF/*.pdf

[thodson-usgs]

My justification being that git stores the entire file for each commit---not a diff--- so a careless user will commit numerous pdfs during the lifecycle of a document, bloating the repo. But I also recognize the convenience of having the pdf accessible via github.