Docs on publishing of header-only C++ libs (include in node c++ modules)

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## rational

We are increasingly working on small node C++ modules that depend on header-only C++ libs.
Node.js's [`Nan` module](https://github.com/nodejs/nan) has pioneered a best practice for how to include header only C++ libs in node C++ addons by simply adding a `package.json` and `include_dirs.js` file to the root of the project. I think we should follow the lead of how`Nan` does this and package all our C++ header only libs in npm as well as mason. I think publishing in two places is worth it because it makes including the libraries from node modules very easy and consistent with how JS modules work.

## method

Add:

- package.json
- include_dirs.js
- `files` parameter to `package.json` so that only the `include/` directory is included

Example:

```
{
"name": "@mapbox/geometry-hpp",
"version": "0.9.0",
"description": "A C++ header only geometry type library",
"main": "./include_dirs.js",
"repository" : {
"type" : "git",
"url" : "git://github.com/mapbox/geometry.hpp.git"
},
"files": [
"include"
]
}
```

Then test that only the right files are going to be packaged:

```
npm pack
tar -ztvf *tgz
```

^^ that should list just the headers and the default files npm includes like `README.md` and `package.json` etc

Examples:

- https://github.com/mapbox/protozero/pull/59
- https://github.com/osmcode/libosmium/pull/186

Example use case:

- https://github.com/springmeyer/node-osmium-count/blob/master/package.json#L17

Locate the header-only C++ libraries in this repository and compare Nan's package.json and include_dirs.js pattern with the linked examples. Add the requested package metadata for each library, then run npm pack and inspect the tarball to confirm it contains the headers and npm's default files only.

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