Add pdm to non-PyPA project list (#987)

* Add pdm to key_projects

* Update source/key_projects.rst

Co-authored-by: Brian Rutledge <brian@bhrutledge.com>

Co-authored-by: Brian Rutledge <bhrutledge@gmail.com>
Co-authored-by: Brian Rutledge <brian@bhrutledge.com>

Author: frostming

source/key_projects.rst

@@ -464,6 +464,20 @@ multibuild
 Multibuild is a set of CI scripts for building and testing Python :term:`wheels <Wheel>` for
 Linux, macOS, and (less flexibly) Windows. Also see :ref:`cibuildwheel`.
 
+.. _pdm:
+
+pdm
+===
+
+`Docs <https://pdm.fming.dev/>`__ |
+`GitHub <https://github.com/pdm-project/pdm/>`__ |
+`PyPI <https://pypi.org/project/pdm>`__
+
+PDM is a modern Python package manager with :pep:`582` support. It installs and
+manages packages in a similar way to ``npm`` that doesn't need to create a
+:term:`virtual environment` at all. It also uses :term:`pyproject.toml` to store
+project metadata as defined in :pep:`621`.
+
 .. _pex:
 
 pex