Documentation for implementation limits and related design choices

[grisumbras]

Documentation would benefit from the
info on implementation-limits-related design choices.

• Guaranteed roundtrip for value -> serialized text -> value (but note that infinity and NaN can be in the original value and do not currently serialize properly).
• JSON inputs where number values cannot be represented losslessly in
  uint64_t, int64_t and double, may render different values when parsed and
  then serialized back, and for extremely big number values can even fail to
  parse (document supported number ranges).
• Whatever JSON output you can produce with this library, we guarantee it
  can be passed by any common JSON implementation (probably also based on
  uint64_t + int64_t + double implementation) (this hardly can be anything specific, we can only present it as a goal of the library).

Also

• Configuration macros that affect implementation limits
• Hard-coded/default implementation limits