Write "s3cmd" setup document

[yosukehara]

• Related issues:
  • https://github.com/leo-project/leofs/issues/19
  • https://github.com/leo-project/leofs/issues/20

[yosukehara]

• DONE
  • https://github.com/leo-project/leofs_docs/compare/2408533f83...90e44d156a
  • https://github.com/leo-project/leofs_docs/compare/90e44d156a...781f904149
  • https://github.com/leo-project/leofs_docs/commit/a2352f8b826d9113c0ead583fddf6c68dc6ba439
  • https://github.com/leo-project/leofs_docs/commit/257a0dfabb12dc6d5b3f9073fc43764a5b23b113