📝 ppn2ocr: Mention upgrading pip

Installing the ppn2ocr requirements fails when pip is not updated (see issue below),
so mention the issue in the README.

Quoting https://github.com/skvark/opencv-python#frequently-asked-questions:

Q: Pip install fails with ModuleNotFoundError: No module named 'skbuild'?
Since opencv-python version 4.3.0.*, manylinux1 wheels were replaced by manylinux2014 wheels. If your pip is too old, it will try to use the new source distribution introduced in 4.3.0.38 to manually build OpenCV because it does not know how to install manylinux2014 wheels. However, source build will also fail because of too old pip because it does not understand build dependencies in pyproject.toml. To use the new manylinux2014 pre-built wheels (or to build from source), your pip version must be >= 19.3. Please upgrade pip with pip install --upgrade pip.

README.md

@@ -69,7 +69,7 @@ ppn2ocr
 The `ppn2ocr` script produces a METS file with the best images for a given
 document in the State Library Berlin (SBB)'s digitized collection. 
 
-Install it, preferably in a venv:
+Install it with an up-to-date pip (otherwise this will fail due to [a opencv-python-headless build failure](https://github.com/skvark/opencv-python#frequently-asked-questions)):
 ~~~
 pip install -r ~/devel/my_ocrd_workflow/requirements-ppn2ocr.txt
 ~~~