Adds a Forum

If you have trouble with creating a Forum, try creating Forums via the web interface, and then access those Forums via the API. This way you can see examples of the fields and sub-elements attached to the Forum. A common source of problems during write/update request is caused when users omit required fields, or insert invalid data in a field (e.g. reference a CATEGORY_ID that does not exist in the user's Insightly instance.