Clarify governance of arrow crate

[alamb]

Which issue does this PR close?

N/A

Rationale for this change

I felt the first line of the arrow crate description on crates.io was a little vague about what "official" meant.

https://crates.io/crates/arrow

What changes are included in this PR?

Clarify docs

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #1030 (e2176f1) into master (e0abda2) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1030      +/-   ##
==========================================
- Coverage   82.31%   82.31%   -0.01%     
==========================================
  Files         168      168              
  Lines       49031    49031              
==========================================
- Hits        40359    40358       -1     
- Misses       8672     8673       +1     

Impacted Files	Coverage Δ
arrow/src/array/transform/mod.rs	84.96% <0.00%> (-0.28%)	⬇️
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (ø)
arrow/src/datatypes/datatype.rs	66.38% <0.00%> (+0.42%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e0abda2...e2176f1. Read the comment docs.