Suggest to use json validator for modders. (cataclysmbnteam#1623)

* Suggest to use json validator for modders.

* Update doc/MODDING.md

Co-authored-by: Olanti <olanti-p@yandex.ru>

doc/MODDING.md

@@ -217,6 +217,8 @@ In game, that appears like this:
 
 Many editors have features that let you track `{ [` and `] }` to see if they're balanced (ie, have a matching opposite); These editors will also respect escaped characters properly. [Notepad++](https://notepad-plus-plus.org/) is a popular, free editor on Windows that contains this feature.  On Linux, there are a plethora of options, and you probably already have a preferred one 🙂
 
+You may also want to use some online or offline JSON validator, e.g. https://dev.narc.ro/cataclysm/format.html , or `json_formatter` bundled with every release of the game (just drag-and-drop the .json file on it).
+
 ## Addendum
 <!-- I really don't know if this should be here or not. Please let me know. -->
 ### No Zombie Revival