Update safety disclaimer

[tustvold]

I think we're in a much better place than when this was originally written, and I think we now have a pretty solid story here. This PR updates the README.md, which in turn will update the crates.io page, to articulate that story

[codecov-commenter]

Codecov Report

Merging #1837 (2e45eb2) into master (bd33489) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1837      +/-   ##
==========================================
- Coverage   83.48%   83.48%   -0.01%     
==========================================
  Files         201      201              
  Lines       56838    56838              
==========================================
- Hits        47451    47450       -1     
- Misses       9387     9388       +1     

Impacted Files	Coverage Δ
arrow/src/datatypes/datatype.rs	65.42% <0.00%> (-0.38%)	⬇️
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (ø)

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[alamb]

LGTM -- thanks @tustvold