readme - info about restart

We lost a few minutes to try to run this plugin, with errors:

```
org.elasticsearch.common.settings.NoClassSettingsException: Failed to load class with value [wikipedia]
	at org.elasticsearch.river.RiverModule.loadTypeModule(RiverModule.java:87)
	at org.elasticsearch.river.RiverModule.spawnModules(RiverModule.java:58)
	at org.elasticsearch.common.inject.ModulesBuilder.add(ModulesBuilder.java:44)
	at org.elasticsearch.river.RiversService.createRiver(RiversService.java:135)
	at org.elasticsearch.river.RiversService$ApplyRivers$2.onResponse(RiversService.java:270)
	at org.elasticsearch.river.RiversService$ApplyRivers$2.onResponse(RiversService.java:264)
	at org.elasticsearch.action.support.TransportAction$ThreadedActionListener$1.run(TransportAction.java:93)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: wikipedia
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at org.elasticsearch.river.RiverModule.loadTypeModule(RiverModule.java:73)
	... 9 more
```

The resolution was to simply restart the elastic search service. Maybe this small change to readme.md will be usefull for other :)