Add documentation links to example swupdate.service

Signed-off-by: Christian Storm <christian.storm@siemens.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>

recipes-support/swupdate/swupdate/swupdate.service

@@ -1,5 +1,7 @@
 [Unit]
-Description=swupdate
+Description=SWUpdate daemon
+Documentation=https://github.com/sbabic/swupdate
+Documentation=https://sbabic.github.io/swupdate
 
 [Service]
 ExecStartPre=-/usr/bin/swupdate-env