add note about duration

Author: zeitlinger

span-stacktrace/README.md

@@ -33,6 +33,9 @@ file_format: 1.0-rc.1
 tracer_provider:
   processors:
     - experimental-stacktrace:
+        min:
+          duration: 10 # minimal duration in ms, default is 5, MUST be an integer
+        filter: my.class.Name # optional, default is to include all spans
 ```
 
 This configuration will register the StackTraceSpanProcessor for all spans.