Added Gloo as an example of an ingress controller (kubernetes#12218)

* Added Gloo as an example of an ingress controller

* put in alphabetical order

content/en/docs/concepts/services-networking/ingress.md

@@ -68,6 +68,7 @@ Additional controllers include:
   provided and supported by Heptio.
 * F5 Networks provides [support and maintenance](https://support.f5.com/csp/article/K86859508)
   for the [F5 BIG-IP Controller for Kubernetes](http://clouddocs.f5.com/products/connectors/k8s-bigip-ctlr/latest).
+* [Gloo](https://gloo.solo.io) is an open-source ingress controller based on [Envoy](https://www.envoyproxy.io) which offers API Gateway functionality with enterprise support from [solo.io](https://www.solo.io).  
 * [HAProxy](http://www.haproxy.org/) based ingress controller
   [jcmoraisjr/haproxy-ingress](https://github.com/jcmoraisjr/haproxy-ingress) which is mentioned on the blog post
   [HAProxy Ingress Controller for Kubernetes](https://www.haproxy.com/blog/haproxy_ingress_controller_for_kubernetes/).