Adding initial EndpointSlice documentation (#16018)

content/en/docs/reference/glossary/endpoint-slice.md

@@ -0,0 +1,19 @@
+---
+title: Endpoint Slice
+id: endpoint-slice
+date: 2018-04-12
+full_link: /docs/concepts/services-networking/endpoint-slices/
+short_description: >
+  A way to group network endpoints together with Kubernetes resources.
+
+aka:
+tags:
+- networking
+---
+ A way to group network endpoints together with Kubernetes resources.
+
+<!--more-->
+
+A scalable and extensible way to group network endpoints together. These can be
+used as by {{< glossary_tooltip text="kube-proxy" term_id="kube-proxy" >}} to
+establish network routes on each {{< glossary_tooltip text="node" term_id="node" >}}.