Add docs on JSON post, put and patch requests

[JonMidhir]

Addresses https://github.com/jnunemaker/httparty/issues/743

This change adds docs regarding how to send data in JSON format. Users are often^[1][2] on how to format JSON data to use in POST, PUT and PATCH requests. This adds simple examples on how to do so.

1: https://stackoverflow.com/questions/7455744/post-json-to-api-using-rails-and-httparty 2: https://github.com/jnunemaker/httparty/issues/743