[DOC] Add section Methods for Working with JSON

[BurdetteLamar]

The linked methods are weakly documented at present, but stronger doc is already in a PR over at JSON.

[mrkn]

@peterzhu2118 Wait, why did you merge this without the maintainer confirmation?

These are methods added by json.gem, right? So, these description should put in the documentation of json.gem.

[peterzhu2118]

Sorry, I should not have merged without maintainer's approval.

These are methods added by json.gem, right? So, these description should put in the documentation of json.gem.

Yes, but since they add these methods to the BigDecimal class, I don't think we can put the documentation in the JSON gem due to RDoc limitations. For example, in the documentation BigDecimal#to_json appears as a method that is in BigDecimal.

[mrkn]

Alright, I understand that we need to include them in this repository. Therefore, I think it's important to note that 'json/add/bigdecimal' is provided by the json.gem. What are your thoughts?

[peterzhu2118]

I opened #278 to add this.