add parsing examples

[ad34]

Would be nice to have some parsing example , I suppose if I have an object containing a string with "route" key I can get the data like this:

msg["route"].string_value()

but having a complete example would be great.

[ar90n]

Hi ad34.

Sorry for my late reply.
Your advice is great !
I will add an example like yours to README.md and create a cpp file which include plain serialization and deserialization.

[kenkit]

also please show how serialize std::map to file