[yt-dev] Issue #842: Document derived_fields in 3.0 (yt_analysis/yt)

New issue 842: Document derived_fields in 3.0 https://bitbucket.org/yt_analysis/yt/issue/842/document-derived_fields-in-30 Cameron Hummels: Currently the derived_field docs are out of sync with the unit changes in 3.0. Specifically, using a `convert_function` fails when you use it as specified in: http://yt-project.org/docs/dev-3.0/analyzing/creating_derived_fields.html