python: Add no-install usage instructions

It's not encouraged, but it's legitimate to want to know how to do.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-id: 20210629214323.1329806-7-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/README.rst

@@ -37,6 +37,34 @@ See `Installing packages using pip and virtual environments
 for more information.
 
 
+Using these packages without installing them
+--------------------------------------------
+
+These packages may be used without installing them first, by using one
+of two tricks:
+
+1. Set your PYTHONPATH environment variable to include this source
+   directory, e.g. ``~/src/qemu/python``. See
+   https://docs.python.org/3/using/cmdline.html#envvar-PYTHONPATH
+
+2. Inside a Python script, use ``sys.path`` to forcibly include a search
+   path prior to importing the ``qemu`` namespace. See
+   https://docs.python.org/3/library/sys.html#sys.path
+
+A strong downside to both approaches is that they generally interfere
+with static analysis tools being able to locate and analyze the code
+being imported.
+
+Package installation also normally provides executable console scripts,
+so that tools like ``qmp-shell`` are always available via $PATH. To
+invoke them without installation, you can invoke e.g.:
+
+``> PYTHONPATH=~/src/qemu/python python3 -m qemu.qmp.qmp_shell``
+
+The mappings between console script name and python module path can be
+found in ``setup.cfg``.
+
+
 Files in this directory
 -----------------------