Manage variables via systemd override config (#4)

Use systemd override configuration to avoid putting sensitive informations in the service file (and so in the git repository).

- add a makefile to install and uinstall the service
- add some documentation to README.md

See #1

Makefile

@@ -0,0 +1,13 @@
+systemd_path := /etc/systemd/system
+install:
+	cp borgbackup.* $(systemd_path)/
+	chmod a+x $(systemd_path)/borgbackup.*
+	systemctl edit borgbackup.service
+	systemctl enable borgbackup.timer
+	systemctl start borgbackup.timer
+
+uninstall:
+	systemctl disable borgbackup.timer
+	rm -rvf $(systemd_path)/borgbackup.service
+	rm -rvf $(systemd_path)/borgbackup.timer
+	rm -rvf /etc/systemd/system/borgbackup.service.d