add select clause so example code compiles (#2097)

Author: jamesmanning

README.md

@@ -51,7 +51,8 @@ Rx elevates the support for live streams of information to the same level as we
 ```cs
 var bigTrades =
     from trade in trades
-    where trade.Volume > 1_000_000;
+    where trade.Volume > 1_000_000
+    select trade;
 ```
 
 This uses C#'s LINQ feature to filter `trades` down to those entities with a volume greater than one million. This query expression syntax is just a shorthand for method calls, so we could also write it this way: