Document running phosh at system boot

 - What about phosh package in the running session?
 - What to do about lingering gnome-session on systemd unit stop?

README.md

@@ -24,15 +24,38 @@ way to get going is to do the following:
 
 
 ## Running
-When running from the source tree start *rootston*. Then start *phosh*
-using:
+### Running from the source tree
+When running from the source tree start the compositor *rootston*. Then start
+*phosh* using:
 
     _build/run
 
-or in one command:`
+or in one command:
 
     ../wlroots/_build/rootston/rootston -E _build/run -C ./rootston.ini
 
-This will make sure the gsettings schema is found, there's enough of a GNOME
-session running an the some of the mutter DBus API is stubbed so
-gnome-settings-manager can work.
+This will make sure the needed gsettings schema is found.
+
+### Running from the Debian packages
+If installed via the Debian packages you can also run phosh via gnome-session.
+It ships a file in /usr/share/gnome-session/sessions so you can bring up a
+session using
+
+    gnome-session --disable-acceleration-check phosh
+
+This assumes you have the compositor already running. If you want to start
+phosh at system boot there's a systemd unit file in */lib/systemd/system/phosh*
+which is disabled by default:
+
+    systemctl enable phosh
+    systemctl start phosh
+
+This runs *phosh* as user *purims* (which needs to exist). If you don't have a
+user *purism* and don't want to create on you can make systemd run *phosh* as
+any user by using an override file:
+
+    $ cat /etc/systemd/system/phosh.service.d/override.conf
+    [Service]
+    User=<your_user>
+    WorkingDirectory=<your_home_directory>
+