In systemd: added log files and changed workingdir (#287)

* coredns log configuration for logrotate

In order to not let the log files (wrote in the systemd file) grew too much, we should move this "coredns-log.conf" file inside /etc/logrotate.d/
In this case I set 50MB of logs, splitted in 5 files, rotation occurs every time on file reaches 10MB in size, I also said to compress the file after the rotation, so even less space should be occupied,
you can change this settings with whatever configuration you prefer. :)

* Update README.md

updated readme with instruction with where to move the configuration file for coredns-log.conf

* Update README.md

* changed workingdir and added log files

sometimes using ~ in the workingdir does not work
Also I'd like to print a basic log of what's happening

* update logs in coredns.service

Changed StandardOutput and StandardError from file: to append:
(so when coredns restart logs continue to be printed)

systemd/coredns-log.conf

@@ -0,0 +1,13 @@
+/var/log/coredns.log {
+  rotate 5
+  size 10M
+  compress
+  notifempty
+}
+
+/var/log/coredns.err.log {
+  rotate 5
+  size 10M
+  compress
+  notifempty
+}