README.md: make clear that libpaper is rquired to use PSUtils

Author: rrthomas

README.md

@@ -36,14 +36,17 @@ The easiest way to install PSUtils is from PyPI, the Python Package Index:
 
 (Note the PyPI package name!)
 
+PSUtils requires libpaper, which allows named paper sizes to be used and
+configured:
+
+libpaper: https://github.com/rrthomas/libpaper
+
+Unfortunately, `pip` cannot install libpaper for you, but you might be able to install it with `brew` or from other package managers. Otherwise, you can install libpaper from source (see the link above).
 
 ## Installation from source or git
 
 PSUtils requires Python 3.9 or later, a handful of Python libraries (listed
-in `pyproject.toml`, and automatically installed by the build procedure),
-and libpaper, which allows named paper sizes to be used and configured:
-
-libpaper: https://github.com/rrthomas/libpaper
+in `pyproject.toml`, and automatically installed by the build procedure).
 
 In the source directory: `python -m build` (requires the `build` package to
 be installed).