[Rust] [Parquet] Add README

[asfimport]

We should augment the existing README with information about the newly added Parquet support.

Reporter: Chao Sun / @sunchao

Related issues:

• [Rust] Add README and executable files to parquet (is duplicated by)

_{Note: This issue was originally created as ARROW-4062. Please see the migration documentation for further details.}

[asfimport]

Chao Sun / @sunchao:
Resolved by ARROW-4160.