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ARROW-4029: [C++] Exclude headers with 'internal' from installation. Document header file conventions in README

In reviewing what usages of the `install` command we have, I added a helper function to add `.pc` files to reduce code duplication

Author: Wes McKinney <wesm+git@apache.org>

Closes #3176 from wesm/ARROW-4029 and squashes the following commits:

f5b3811fc <Wes McKinney> Exclude headers with 'internal' from installation. Document in README. Add function to reduce code duplication in adding pkg-config files