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ARROW-484: Revise README to include more detail about software components

Also closes #14.

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #286 from wesm/ARROW-484 and squashes the following commits:

8c6acf6 [Wes McKinney] Tweak description of data containers
ec4b95e [Wes McKinney] Generalize note about binary formats
3d31644 [Wes McKinney] Typos
57b8bf5 [Wes McKinney] Revise README to include more detail about software components