systemd requirements tweak.

`Requires=redis.service` should be commented unless the service is
started as a privileged user.

Also added some comments on how the systemd service usually works.

examples/systemd/platypush.service

@@ -1,7 +1,25 @@
+# Platypush systemd service file.
+#
+# It is advised to install Platypush through a supported package manager or a
+# Platypush Docker container.
+#
+# In such case, the service file should be automatically installed and configured,
+# and Platypush can be started either via `[sudo] systemctl start platypush`
+# (privileged start, usually with a dedicated `platypush` user) or
+# `systemctl --user start platypush` if you want to run it through a generic user.
+#
+# In the first case, the working directory would usually be
+# `/var/lib/platypush`, and the configuration will be stored under `/etc/platypush`.
+#
+# You can however also copy this service configuration either to
+# `/etc/systemd/system/platypush.service` or
+# `~/.config/systemd/user/platypush.service`.
+
 [Unit]
 Description=Platypush - A general-purpose platform for automation
 After=network.target redis.service
-Requires=redis.service
+# Uncomment this line if you're running the service as a privileged user
+# Requires=redis.service
 
 [Service]
 ExecStart=/usr/bin/platypush