No simple way to author and share flow packages.

Documentation does not seem to provide much insight on what is an idiomatic way to write packages like what file extension to use how to build and publish to allow others consume your packages without needing to customize their configurations.

I will just assume that setup used by [graphql-js](https://github.com/graphql/graphql-js) is an idiomatic way to go about it:
- Author source in `src/` directory as `module.js` files.
- Use babel to transpile to plain JS.
- Publish to npm.

Problem with such setup is that users of my library loose all of the type information that original source contained. There are ways to workaround it but they either require flow configuration on the consumer side or non trivial build step from the publisher side.
## Proposal

I would like to propose following things to improve current situation & grow number of flow typed packages distributed over npm.
1. Documentation should certainly include a guide on how to author and publish flow typed package.
2. I would suggest to make `module.flow` an idiomatic file extension for flow typed files. For following reasons:
1. Flow extends JS syntax so sharing same file extensions does not really makes sense.
2. If you author files flow code in `module.js` you can't really generate compiled JS files alongside of them.
3. In 0.19 introduced useful feature, that allows putting `module.js.flow` along the side of `module.js` in which case flow would give precedence to `module.js.flow` over `module.js` when importing `./module`. Unfortunately no attempt to load `module.flow` is made. It would actually make sense to alter algorithm slightly so that flow would try `module.flow` then `module.js.flow` and then `module.js` which in fact would match node's algorithm more closely which attempts to load `module` and then `module.js`.

This would enable users to author their packages in `module.flow` files and then compile all `module.flow` files to `module.js` files along the side them. Publishing such package into npm also would preserve all the type information for consumers, since flow would first look for `dep/module.flow` (which will be an original source) before `dep/module.js`.

Note: Technically you could achieve more or less same even if your flow code was authored in `module.js` files but that would require copying and renaming them to `module.js.flow` which as far as I can tell has no trivial cross platform solution & requiring extra scripts isn't great. Another alternative is to just author code as `module.js.flow` and generate `module.js` files, it's just authoring in `.js.flow` file extension seems awkward.