Add unpaper as another pre-processing step

docs/requirements.rst

@@ -10,11 +10,13 @@ should work) that has the following software installed on it:
 * `GNU Privacy Guard`_
 * `Tesseract`_
 * `Imagemagick`_
+* `unpaper`_
 
 .. _Python3: https://python.org/
 .. _GNU Privacy Guard: https://gnupg.org
 .. _Tesseract: https://github.com/tesseract-ocr
 .. _Imagemagick: http://imagemagick.org/
+.. _unpaper: https://www.flameeyes.eu/projects/unpaper
 
 Notably, you should confirm how you access your Python3 installation.  Many
 Linux distributions will install Python3 in parallel to Python2, using the names