sources/ostree: add documentation and schema

Add a brief documentation text and its JSON schema so that osbuild
can verify options org.osbuild.ostree source entries.

sources/org.osbuild.ostree

@@ -1,4 +1,11 @@
 #!/usr/bin/python3
+"""Fetch OSTree commits from an repository
+
+Uses ostree to pull specific commits from (remote) repositories
+at the provided `url`. Can verify the commit, if one or more
+gpg keys are provided via `gpgkeys`.
+"""
+
 
 import json
 import os
@@ -7,6 +14,45 @@ import subprocess
 import uuid
 
 
+SCHEMA = """
+"additionalProperties": false,
+"properties": {
+  "commits": {
+    "description": "The commits to fetch indexed their checksum",
+    "type": "object",
+    "additionalProperties": false,
+    "patternProperties": {
+      "[0-9a-f]{5,64}": {
+        "type": "object",
+        "additionalProperties": false,
+        "required": ["remote"],
+        "properties": {
+          "remote": {
+            "type": "object",
+            "additionalProperties": false,
+            "required": ["url"],
+            "properties": {
+              "url": {
+                "type": "string",
+                "description": "URL of the repository."
+              },
+              "gpgkeys": {
+                "type": "array",
+                "items": {
+                  "type": "string",
+                  "description": "GPG keys to verify the commits"
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+  }
+}
+"""
+
+
 def ostree(*args, _input=None, **kwargs):
     args = list(args) + [f'--{k}={v}' for k, v in kwargs.items()]
     print("ostree " + " ".join(args), file=sys.stderr)