Add descriptions.tsv to schema.

src/schema/objects/files.yaml

@@ -126,3 +126,8 @@ stimuli:
     A directory to store any stimulus files used during an experiment.
     See the [relevant section](SPEC_ROOT/modality-specific-files/task-events.md#stimuli-directory)
     for more information.
+descriptions:
+  display_name: Descriptions Definitions
+  file_type: regular
+  description: |
+    A TSV file describing labels found for the `desc` entity in a Derivatives dataset.

src/schema/rules/files/deriv/tables.yaml

@@ -0,0 +1,7 @@
+---
+descriptions:
+  level: optional
+  stem: descriptions
+  extensions:
+    - .tsv
+    - .json

src/schema/rules/tabular_data/derivatives/common_derivatives.yaml

@@ -10,3 +10,12 @@ SegmentationLookup:
     color: optional
     mapping: optional
   index_columns: [index]
+
+Descriptions:
+  selectors:
+    - path == "descriptions.tsv"
+  columns:
+    desc_id: required
+    description: required
+  index_columns: [desc_id, description]
+  additional_columns: allowed