Update configuration.md

configuration.md

 ### Configuration
+## References
+Available options can be found directly on the source code
+```
+https://github.com/ceph/ceph/tree/master/src/common/options
+```
+Or querying the daemon
+```
+ceph daemon	<type.id>	config <diff|get|set|show>
+"config diff": "dump diff of current config and default config"
+"config get": "config get <field>: get the config value"
+"config set": "config set <field> <val> [<val> ...]: set a config variable"
+"config show": "dump current config settings"
+```
+Example: Which	options	exist	for	osd scrubbing?
+```
+# ceph daemon osd.0 config show | grep scrub
+"osd_scrub_thread_timeout": "60",
+"osd_scrub_thread_suicide_timeout": "300",
+"osd_scrub_finalize_thread_timeout": "600",
+"osd_scrub_invalid_stats": "true",
+"osd_max_scrubs": "1",
+```
 ## OSD memory Configuration
 # Changing paramenter of a running service
 ```