Add "Admission Controller" to the glossary (#15196)

This gives a brief description for the admission controller and links
to the main docs for them.

content/en/docs/reference/glossary/admission-controller.md

@@ -0,0 +1,22 @@
+---
+title: Admission Controller
+id: admission-controller
+date: 2019-06-28
+full_link: /docs/reference/access-authn-authz/admission-controllers/
+short_description: >
+  A piece of code that intercepts requests to the Kubernetes API server prior to persistence of the object.
+
+aka:
+tags:
+- extension
+- security
+---
+A piece of code that intercepts requests to the Kubernetes API server prior to persistence of the object.
+
+<!--more-->
+
+Admission controllers are configurable for the Kubernetes API server and may be “validating”, “mutating”, or
+both. Any admission controller may reject the request. Mutating controllers may modify the objects they admit;
+validating controllers may not.
+
+* [Admission controllers in the Kubernetes documentation](/docs/reference/access-authn-authz/admission-controllers/)