Add information about CustomLoadBalancer in the Rider documentation

Eric we have changed a bit how you use custom load balancers in Camel / Rider.

There is now a <custom> tag you use, so its just like using any other load balancer. The ref attribute on the <loadBalance> tag has been @deprecated and kept for backwards comp.

I added some documentation at the Apache side you can use for inspiration
https://cwiki.apache.org/confluence/display/CAMEL/Load+Balancer