Add reference to Spring reference doc on Modules

Since the main example here is using the `spring-context` module,
Spring developers should be aware of all the other existing Spring
modules and their artifacts names.

A link to the corresponding section in the reference documentation is
added.

Issue: SPR-11534

index.html

@@ -53,6 +53,8 @@ deployment environments.
 * Foundational support for JDBC, JPA, JMS
 * Much more...
 
+All avaible features, modules and their maven/gradle artifact names are described in [the Modules section of the reference documentation](http://docs.spring.io/spring-framework/docs/current/spring-framework-reference/html/overview.html#overview-modules).
+
 <span id="quick-start"></span>
 ## Quick Start