Add link to docs zip

Closes gh-16798

docs/modules/ROOT/pages/index.adoc

@@ -1,5 +1,10 @@
 = Spring Security
 
+[NOTE]
+====
+Spring Security's documentation can be http://docs.spring.io/spring-security/reference/spring-security-docs.zip[downloaded] as a zip file.
+====
+
 Spring Security is a framework that provides xref:features/authentication/index.adoc[authentication], xref:features/authorization/index.adoc[authorization], and xref:features/exploits/index.adoc[protection against common attacks].
 With first class support for securing both xref:servlet/index.adoc[imperative] and xref:reactive/index.adoc[reactive] applications, it is the de-facto standard for securing Spring-based applications.