ARROW-2635: [Ruby] Add LICENSE.txt and NOTICE.txt for Apache Arrow Ruby

[kou]

They are for packaging gems. The Apache Arrow Ruby gems only include Apache Arrow Ruby related codes. So top-level LICENSE.txt and NOTICE.txt aren't suitable for it.

[codecov-io]

Codecov Report

Merging #2083 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2083      +/-   ##
==========================================
- Coverage   86.35%   86.33%   -0.03%     
==========================================
  Files         242      230      -12     
  Lines       41184    40292     -892     
==========================================
- Hits        35566    34786     -780     
+ Misses       5618     5506     -112

Impacted Files	Coverage Δ
cpp/src/plasma/store.cc	88.76% <0%> (-0.46%)	⬇️
rust/src/list.rs
rust/src/lib.rs
rust/src/memory_pool.rs
rust/src/builder.rs
rust/src/array.rs
rust/src/datatypes.rs
rust/src/buffer.rs
rust/src/error.rs
rust/src/list_builder.rs
... and 4 more

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[wesm]

+1