Add example readme

[alamb]

Which issue does this PR close?

Related to #1613

Having concise descriptions of examples will help people find them

What changes are included in this PR?

README.md file in arrow / examples directory

See rendered version: https://github.com/alamb/arrow-rs/tree/alamb/examples_doc/arrow/examples

Are there any user-facing changes?

moar docs!

[codecov-commenter]

Codecov Report

Merging #1615 (4d1ef62) into master (5374289) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 4d1ef62 differs from pull request most recent head 01caa0a. Consider uploading reports for the commit 01caa0a to get more accurate results

@@            Coverage Diff             @@
##           master    #1615      +/-   ##
==========================================
- Coverage   82.90%   82.89%   -0.01%     
==========================================
  Files         193      193              
  Lines       55464    55464              
==========================================
- Hits        45980    45978       -2     
- Misses       9484     9486       +2     

Impacted Files	Coverage Δ
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.69%)	⬇️
parquet/src/encodings/encoding.rs	93.37% <0.00%> (-0.19%)	⬇️
arrow/src/array/transform/mod.rs	86.57% <0.00%> (+0.11%)	⬆️
arrow/src/datatypes/datatype.rs	66.80% <0.00%> (+0.39%)	⬆️

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[viirya]

Missing right "`"

[viirya]

And, maybe builders.rs? To be consistent with others.

[alamb]

in 6851adf

[alamb]

Thanks @viirya