Update README.md

add Process.wait to example script, fix the zombie process problem

README.md

@@ -31,9 +31,10 @@ c = LXC::Container.new('foo')
 c.create('ubuntu') # create a container named foo with ubuntu template
 c.start
 # attach to a running container
-c.attach do
+pid = c.attach do
   LXC.run_command('ifconfig eth0')
 end
+Process.wait(pid)
 c.stop
 c.destroy
 ```