Add comments on app_mode with possible values

conf/defaults.ini

@@ -3,6 +3,7 @@
 # Do not modify this file in grafana installs
 #
 
+# possible values : production, development
 app_mode = production
 
 #################################### Paths ####################################

conf/sample.ini

@@ -3,6 +3,7 @@
 # Everything has defaults so you only need to uncomment things you want to
 # change
 
+# possible values : production, development    
 ; app_mode = production
 
 #################################### Paths ####################################