add flow type for react-intl MessageDescriptor

src/flowtype/npm/react-intl.js

@@ -0,0 +1,14 @@
+export type MessageDescriptor = {
+    // A unique, stable identifier for the message
+    id: string, 
+    
+    // The default message (probably in English)
+    defaultMessage: string,
+    
+    // Context for the translator about how it's used in the UI
+    description?: string | object, 
+};
+
+export type Messages = {
+    [key: string]: MessageDescriptor
+};