Documentation on when/why/how to write `_impl.hpp`

@tmpsantos thanks for working on https://github.com/mapbox/geojson-cpp/pull/24. Now I'd like your help in documenting your work so others can do this again for further projects. Can you write up a quick guide on how you accomplished this, key terms, and why you chose _impl.hpp as an extention (let's standardize on that). Drop your notes into this ticket as a comment please and then we'll integrate into the main readme.

/cc @mapsam @GretaCB

refs https://github.com/mapbox/hpp-skel/issues/3

まず pull request #24 と、参照されている hpp-skel の issue #3 の作業を読んでください。作業がどのように行われたか、主要な用語、そしてなぜ _impl.hpp を標準化すべきなのかを扱う短いガイドを書き、その後、メモをコメントとして追加し、メインの README に統合してください。