Add an ability to add a valueHint to referenceTypes.

Part of #682.

src/components/card-editor.ts

@@ -996,7 +996,7 @@ class CardEditor extends connect(store)(LitElement) {
 		if (map) {
 			val = map[refType] || '';
 		}
-		const newVal = prompt('What do you want the value to be?', val);
+		const newVal = prompt(`What do you want the value to be?${config.valueHint ? '\n' + config.valueHint : ''}`, val);
 		if (newVal == null) return;
 		store.dispatch(addReferenceToCard(cardID, refType, newVal));
 	}

src/types.ts

@@ -716,6 +716,9 @@ export type ReferenceTypeConfigurationMap = {
 		reciprocal? : boolean,
 		//needsReciprocation - if true, then cards that don't reciprocate with a link will be called out.
 		needsReciprocation? : boolean
+		//If set, this will be shown in the prompt for changing the value.
+		//Different reference types have different idiomatic uses.
+		valueHint? : string
 	}
 }