Brief YAML reference

I see a trend in open source projects lately, which is to use TOML [1] as the preferred configuration and metadata format.

In some cases, I believe YAML would actually be more suitable, but there is a lack of a good, understandable reference page for regular users.

The author of a YAML serialization library for Python [2] wrote this brief YAML reference page.

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[1]: https://github.com/toml-lang/toml

[2]: https://github.com/eevee/camel