Add description for JSON representation of BigDecimal

[smcgivern]

In our cloud pricing API, a FixedCharge has a price, which is a BigDecimal. We added a description for this by monkey-patching the Attributor type:

module Attributor
 class BigDecimal
   def self.describe(root = false, example: nil)
     h = super
     h[:description] = "This is a floating point number encoded as a JSON string in order to preserve precision."
     h
   end
 end
end

It seems like it would make sense to have a description by default (it doesn't have to be this one). Does that belong in Attributor or further up the chain (this is a Praxis app)?

@careo and @blanquer interested in what you think.

[blanquer]

yes, this is related to #79 and #168