[yt-dev] [yt_analysis/yt] Document the field_cuts keyword arg for projections. (issue #555)

[Britton Smith]

New issue 555: Document the field_cuts keyword arg for projections.
https://bitbucket.org/yt_analysis/yt/issue/555/document-the-field_cuts-keyword-arg-for

Britton Smith:

Some discussion should be added to the field_cuts section in analyzing/objects section that discusses this specific use with projections.