docs: Add packaging info

This patch adds information on packaging pynitrokey.  The most important
information is the possibility to generate shell completion scripts for
nitropy.

docs/packaging.rst

@@ -0,0 +1,18 @@
+Packaging pynitrokey
+====================
+
+When ``pynitrokey`` is packaged for a distribution or a package manager, we recommend that the package provides both the ``pynitrokey`` Python library and the ``nitropy`` executable.
+Alternatively, the Python library can be packaged as ``pynitrokey`` and the executable as ``nitropy``.
+(Note that we consider splitting ``pynitrokey`` into a library and an executable in a future release.)
+
+Shell completions for Bash, Zsh and Fish can be generated from the ``nitropy`` script::
+
+    $ _NITROPY_COMPLETE=bash_source nitropy > nitropy.bash
+    $ _NITROPY_COMPLETE=zsh_source nitropy > nitropy.zsh
+    $ _NITROPY_COMPLETE=fish_source nitropy > nitropy.fish
+
+For more information, see the `click documentation`_.
+
+.. _click documentation: https://click.palletsprojects.com/en/8.1.x/shell-completion/
+
+Unfortunately, man pages for ``nitropy`` are currently not available and cannot be generated automatically.