SWIFT-1034 Add sorted find example to README (#605)

Author: patrickfreed

README.md

@@ -137,7 +137,11 @@ print(result?.insertedID ?? "") // prints `.int64(100)`
 **Async**:
 ```swift
 let query: BSONDocument = ["a": 1]
-let result = collection.find(query).flatMap { cursor in
+// The `sort` option specifies the order in which results are returned
+// via the cursor. In this case, `["_id": -1]` indicates that the documents will
+// be returned in descending order according to the `_id` field.
+let options = FindOptions(sort: ["_id": -1])
+let result = collection.find(query, options: options).flatMap { cursor in
     cursor.forEach { doc in
         print(doc)
     }
@@ -147,7 +151,11 @@ let result = collection.find(query).flatMap { cursor in
 **Sync**:
 ```swift
 let query: BSONDocument = ["a": 1]
-let documents = try collection.find(query)
+// The `sort` option specifies the order in which results are returned
+// via the cursor. In this case, `["_id": -1]` indicates that the documents will
+// be returned in descending order according to the `_id` field.
+let options = FindOptions(sort: ["_id": -1])
+let documents = try collection.find(query, options: options)
 for d in documents {
     print(try d.get())
 }