Documentation and usage examples

[dtolnay]

https://docs.rs/json5/0.2.1/json5/ is looking pretty bare.

See these pages for some good examples:

• https://docs.rs/serde_json/1.0.32/serde_json/
• https://docs.rs/serde_cbor/0.9.0/serde_cbor/
• https://docs.rs/toml/0.4.8/toml/
• https://docs.rs/avro-rs/0.6.1/avro_rs/

[callum-oakley]

I finally got round to having a stab at some documentation. https://docs.rs/json5 is hopefully a little more useful now!