Document consuming the format better

[Shnatsel]

The feedback from Tom Fay was that implementing support for the format was trivial in Go, but I'm sure we can simplify it further. The docs on parsing are kind of a mess.

It should lay out all the sections (including Mach-O segment, which isn't explicitly documented), link to the JSON schema (#72), and describe the parsing in detail, including the mention of the possibility of cyclic dependencies.

It should also mention rust-audit-info as an alternative - just call it as a subprocess and get the JSON.

[Shnatsel]

Initial docs added: https://github.com/rust-secure-code/cargo-auditable/blob/master/PARSING.md

It would be great to add a sample in some very common language, e.g. Python.

Also, the README needs to be updated to link to it.

[Shnatsel]

Done as of ba813c8