Make promises usage more explicit

Current async example makes assumption that the promise will be resolved somewhere in the code later, but for newbies to this concept it's not clear.

Author: alexeyshockov

README.md

@@ -298,6 +298,9 @@ $promise->then(
 );
 
 echo "I will print out before the promise is fulfilled";
+
+// You can combine multiple promises using \GuzzleHttp\Promise\all() and other functions from the library.
+$promise->wait();
 ```
 
 ## Handling Exceptions