Add documentation in README.md about how to extract all CSS in one file

[Pierre-Do]

Following discussions on the issues, this PR aims at documenting the common use-case of extracting all CSS in one file, similar to what https://github.com/webpack-contrib/extract-text-webpack-plugin does.

Tested in a small project there: https://github.com/Akkenar/CSS-Single-File-Example

[jsf-clabot]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #60 into master will not change coverage.
The diff coverage is n/a.

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[alexander-akait]

Can we add [contenthash] template to avoid problem with cache who use this configuration

[Pierre-Do]

I agree with you, [hash] could also help with the cache, or the hash: true option on HtmlWebpackPlugin if you're using it. I guess it depends on your project setup.

Not quite sure this documentation section is about cache burst though, hence not adding it in the example configuration.

[alexander-akait]

Looks good thanks!