[openstreetmap/openstreetmap-website] Add labels documentation to CONTRIBUTING.md (#3011)

[Tucker Chapman]

Reporters may not be able to add labels but someone looking for an issue to work on may find this documentation helpful. I am okay with the documentation being on the [labels page](https://github.com/openstreetmap/openstreetmap-website/labels) or the CONTRIBUTING.md file. 

A few benefits of putting the documentation here is @lectrician1 and I can make suggestions that only require accepting Pull Requests and anyone looking to contribute will find them when reading CONTRIBUTING.md.

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