add note with clarification how to pass -Dconfig properly; taken from #…

…84 and https://stackoverflow.com/a/31305181/5151861

README.md

@@ -257,6 +257,8 @@ system properties can be used to force a different config source
    it should include the extension, not be a basename
  - `config.url` specifies a URL
 
+ **Note**: you need to pass `-Dconfig.file=path/to/config-file` before the jar itself, e.g. `java -Dconfig.file=path/to/config-file.conf -jar path/to/jar-file.jar`
+
 These system properties specify a _replacement_ for
 `application.{conf,json,properties}`, not an addition. They only
 affect apps using the default `ConfigFactory.load()`