Add instructions on how to limit the scope for action push trigger

[amkrajewski]

• Looking at repos for papers recently published in JOSS, people in the community will almost universally store paper.* files alongside their code, usually in the paper directory.
• The current usage guide suggests running the workflow on every push, regardless of whether it is to paper or src, wasting resources on generating duplicate paper.pdf drafts (in the case described above).
• This small PR adds instructions to the README on modifying the draft-pdf.yml workflow to limit its trigger to pushes or (a) paper directory or (b) specific files.

[tarleb]

Thanks!