Document how to build portable, cross-platform binaries

Nobody has claimed this yet.

Creating C++ binaries portable to a given platform (osx, linux, windows) or platform version (ubuntu precise or trusty) is absolutely feasible but a poorly documented topic. Often our goal at Mapbox is to support the latest C++11 and C++14 features but maintain code portable to older operating systems like Windows 7, OS X 10.8, and Ubuntu Precise, or RHEL 6/7. This is also feasible. For example its possible to run C++14 binaries on AWS lambda (built on ubuntu precise) even though you can't easily compile them on AWS Lambda.

We should document how Mapbox writes and distributes portable C?C++ code to make clear when this approach is advantageous and when it is not. And to ensure the techniques are robust and replicable.

This issue is to track:

- [ ] getting this all documented here
- [ ] Moving the content from https://github.com/mapbox/node-pre-gyp/wiki/External-libraries to this future doc
- [ ] Moving binary portability FAQ from https://github.com/mapbox/node-pre-gyp/wiki/FAQ#frequently-ask-questions-about-node-pre-gyp to here

Start by reviewing the node-pre-gyp External libraries wiki page and the binary portability FAQ linked in the issue. Consolidate the relevant material into the planned documentation, covering when portable C/C++ binaries are advantageous and how Mapbox writes and distributes them. Done means the topic is documented here and both linked sources have been migrated.

Written by the indexing model from the issue text.