Added pool suggestion

Author: WyriHaximus

composer.json

@@ -22,7 +22,8 @@
   "suggest": {
     "api-clients/middleware-cache": "Cache requests with different strategies",
     "api-clients/middleware-log": "Log requests and their context",
-    "api-clients/middleware-oauth1": "Sign requests with oauth1"
+    "api-clients/middleware-oauth1": "Sign requests with oauth1",
+    "api-clients/middleware-pool": "Pool the maximum concurrent requests"
   },
   "autoload": {
     "psr-4": {