Warn about systemd % expansion in unit files

Fixes #101

systemd/varnishncsa.service

@@ -7,6 +7,19 @@ RuntimeDirectory=varnishncsa
 Type=forking
 User=varnishlog
 Group=varnish
+
+# Beware of custom formats! If you wish to use a different format using the
+# -F option for varnishncsa, please note that systemd also uses percent '%'
+# expansion in unit files: you need to double the percent signs in your
+# format string.
+#
+# For example the default format would look like this:
+#
+#     -F '%%h %%l %%u %%t "%%r" %%s %%b "%%{Referer}i" "%%{User-agent}i"'
+#
+# Alternatively you can read the ncsa format from a file using the -f
+# option in your varnishncsa command line.
+
 ExecStart=/usr/bin/varnishncsa -a -w /var/log/varnish/varnishncsa.log -D
 ExecReload=/bin/kill -HUP $MAINPID