Add comment explaining min and max values of RRD

rpimonitor/default.conf

@@ -56,6 +56,13 @@
 #    will be stored at every extraction. 
 #    Ref http://oss.oetiker.ch/rrdtool/doc/rrdcreate.en.html for RRD 
 #    parameter description.
+#  dynamic.<dynamic data id>.min=<minimal value acceptable in RRD>
+#  dynamic.<dynamic data id>.max=<miximal value acceptable in RRD>
+#    If a value extracted by is less than minimal of greater than maximal
+#    value, it will be stored int RRD database as unknown.
+#    These limits are usefull to handle counter that fall down to 0 when 
+#    they reach their limit (Ex: network graphs)
+# 
 #  Note: Static values are accessible for the post processing using the 
 #  variable $this->{'static'}->{'static_data_name'} and can be used.
 #  You can refer to swap data extraction to see an example.