forgot to add collection

Signed-off-by: chaosinthecrd <[email protected]>

docs/concepts/collection.md

@@ -0,0 +1,58 @@
+# Collection
+
+Witness enables users to generate a wide variety of attestation predicates (arbitrary metadata about a subject artifact, with a type-specific schema) through the use of attestors. For each `witness run`, multiple attestors can be specified and therefore multiple predicates can be
+generated as an output. Witness correlates each `run` invocation to a "step" in an artifacts supply-chain lifecycle (the name of which is determine by the `--step` flag). Witness therefore needs a way of bundling these predicates together to form a complete representation of that specific step, but also to avoid the repeated process of signing multiple statements. The `Collection` object is a predicate type that achieves this.
+
+## Schema
+```json
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://github.com/in-toto/go-witness/attestation/collection",
+  "$ref": "#/$defs/Collection",
+  "$defs": {
+    "Collection": {
+      "properties": {
+        "name": {
+          "type": "string"
+        },
+        "attestations": {
+          "items": {
+            "$ref": "#/$defs/CollectionAttestation"
+          },
+          "type": "array"
+        }
+      },
+      "additionalProperties": false,
+      "type": "object",
+      "required": [
+        "name",
+        "attestations"
+      ]
+    },
+    "CollectionAttestation": {
+      "properties": {
+        "type": {
+          "type": "string"
+        },
+        "attestation": true,
+        "starttime": {
+          "type": "string",
+          "format": "date-time"
+        },
+        "endtime": {
+          "type": "string",
+          "format": "date-time"
+        }
+      },
+      "additionalProperties": false,
+      "type": "object",
+      "required": [
+        "type",
+        "attestation",
+        "starttime",
+        "endtime"
+      ]
+    }
+  }
+}
+```