doc: first_day is depecated

Author: alexander-mart

lib/index.js

@@ -469,7 +469,7 @@ class GitStats {
                     theme: options.theme
                   , start: options.start
                   , end: options.end
-                  , firstDay: options.firstDay
+                  , firstDay: options.firstDay // [DEPRECATED] https://github.com/IonicaBizau/git-stats/issues/121
                   , cal: cal
                   , raw: options.raw
                 }
@@ -661,6 +661,7 @@ GitStats.DEFAULT_CONFIG = {
     // This defaults in library
   , path: undefined
 
+    // [DEPRECATED] https://github.com/IonicaBizau/git-stats/issues/121
     // This defaults in cli-gh-cal
   , first_day: undefined