contrib/systemd: Added note on common issues with systemd and asterisk

contrib/systemd: Added note on common issues with systemd and asterisk

With newer version of linux /var/run/ is a symlink to /run/ that has
been turned into tmpfs.

Added note that if asterisk has to bind to a specific IP that
systemd has to wait until the network is up.

Added note on how to make sure that the environment variable
HOSTNAME is included.

ASTERISK-29216
Reported by: Mark Petersen
Tested by: Mark Petersen

Change-Id: Ib3e560655befd3e99eec743687144f5569533379