Support and document how to publish modules using Linaria to NPM

## Background

Linaria is probably the best option to choose among CSS in JS libraries if you are making a component library which you want to publish on NPM.

- It produces vanilla CSS files, which means anyone can consume them
- It doesn't have a runtime overhead, so regardless what solution the user uses for CSS, this is not going to another CSS in JS lib to the mix

Today I was trying to publish one of my libraries to NPM which uses Linaria but hit a couple of roadblocks. I hope we can address them.

## Approaches
- Use `single: true` and ask users to take care of loading the CSS file
- Use `single: false` and require a bundler like webpack to load the CSS files
- Publish the source to npm and ask users to transpile their `node_modules` with Babel

## Blockers

- The inserted imports are absolute paths, which make it impossible to share the stylesheets among different machines
- The class names might not be unique because they are based on developer's folder structure, so if two people named "Dave" have same folder structure, e.g. `Users/Dave/projects/components`, but have libraries with different names, the class names might clash

## Proposal

- [x] Convert imports paths to be relative to the file, pretty-straightforward (#180)
- [ ] It's impossible to generate globally unique names, but we can have some logic so that the hashes include the package name, for example: instead of `src/App.js`, we use `awesome@version~src/App.js` to generate the hash, where `awesome` is the name of the package and `version` is the version of package we read from `package.json` if it exists, this assumes package names are globally unique. This won't work properly in some scenarios where the styles changed but version number didn't (e.g. installing from git), but it's impossible/extremely hard to do it.

Currently, I only need the first one, which should be easy to fix. Appreciate your thoughts.

We also need to document the process of publishing libraries using Linaria to NPM.