docs: clarified systemd installation instructions in init.md for Ubuntu users.

Added a note to init.md to clarify the .service copy path for newer versions of Ubuntu because it differs from the described copy path.

Also noted what version of Ubuntu switched to systemd for the default system init.

Author: DaveFromBinary

doc/init.md

@@ -84,6 +84,8 @@ Installing this .service file consists of just copying it to
 To test, run `systemctl start bitcoind` and to enable for system startup run
 `systemctl enable bitcoind`
 
+NOTE: When installing for systemd in Debian/Ubuntu the .service file needs to be copied to the /lib/systemd/system directory instead.
+
 ### OpenRC
 
 Rename bitcoind.openrc to bitcoind and drop it in /etc/init.d.  Double
@@ -93,6 +95,8 @@ check ownership and permissions and make it executable.  Test it with
 
 ### Upstart (for Debian/Ubuntu based distributions)
 
+Upstart is the default init system for Debian/Ubuntu versions older than 15.04. If you are using version 15.04 or newer and haven't manually configured upstart you should follow the systemd instructions instead.
+
 Drop bitcoind.conf in /etc/init.  Test by running `service bitcoind start`
 it will automatically start on reboot.