Support URL on retribution means

schemas/0.1/example.json

@@ -22,6 +22,7 @@
       "id": "freetips",
       "name": "Tip me on freetips",
       "weight": 2,
+      "url": "https://freetips.com/@Alice",
       "summary": "This is the best place to tip me"
     }
   ]

schemas/0.1/schema.json

@@ -42,7 +42,7 @@
   },
   "definitions": {
     "mean": {
-      "description": "A single retribution mean",
+      "description": "A single retribution mean. The type and id must form a unique combination",
       "type": "object",
       "required": ["provider", "id", "weight"],
       "minItems": 1,
@@ -68,6 +68,9 @@
         "name": {
           "type": "string"
         },
+        "url": {
+          "type": "string"
+        },
         "summary": {
           "type": "string"
         },