Add a note on rust compiler testing and compatibility

[alamb]

Rationale for this change

@jhorstmann noticed a change that was introduced in #714 (comment) (and included in the arrow 5.3.0 RC0) that is incompatible with rust 1.52.1 (rust is currently on 1.54.0).

What changes are included in this PR?

Propose explicitly document that arrow and parquet are only tested with the latest stable version of Rust.

Though this maybe also should serve as a way to solicit opinions about how important compatibility with older versions of Rust is (and if anyone would be willing to spend the time to setup CI to make sure that happens)

Are there any user-facing changes?

Different readme

[codecov-commenter]

Codecov Report

Merging #726 (c924015) into master (5a12d97) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #726      +/-   ##
==========================================
+ Coverage   82.50%   82.51%   +0.01%     
==========================================
  Files         168      168              
  Lines       47589    47631      +42     
==========================================
+ Hits        39261    39301      +40     
- Misses       8328     8330       +2     

Impacted Files	Coverage Δ
parquet_derive_test/src/lib.rs	100.00% <0.00%> (ø)
parquet_derive/src/parquet_field.rs	68.86% <0.00%> (+2.69%)	⬆️

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[jimexist]

LGTM