Labels description

[SemenchenkoVitaliy]

As for now, added labels won't have a description unless the request has application/vnd.github.symmetra-preview+json in the Accept header. But since it is an experimental feature and preview features are not supported for production use we need to wait until a stable API for this is released. See https://developer.github.com/v3/issues/labels/#create-a-label and https://developer.github.com/changes/2018-02-22-label-description-search-preview.