Update documentation - How to set number of concurrent fetchers

docker-compose.yml

@@ -47,6 +47,9 @@ services:
   #
   #        Hides the `Referer` header so that monitored websites can't see the changedetection.io hostname.
   #      - HIDE_REFERER=true
+  #        
+  #        Default number of parallel/concurrent fetchers
+  #      - FETCH_WORKERS=10
 
       # Comment out ports: when using behind a reverse proxy , enable networks: etc.
       ports: