Add generic GemFire findByIndexNameAndIndexValue support

Fixes gh-353

docs/src/docs/asciidoc/index.adoc

@@ -257,6 +257,56 @@ Guide when integrating with your own application.
 
 include::guides/httpsession-gemfire-p2p-xml.adoc[tags=config,leveloffset=+3]
 
+[[httpsessoin-gemfire-indexing]]
+==== Using Indexes with GemFire
+
+While best practices concerning the proper definition of indexes that positively impact GemFire's performance is beyond
+the scope of this document, it is important to realize that Spring Session Data GemFire creates and uses indexes to
+query and find Sessions efficiently.
+
+Out-of-the-box, Spring Session Data GemFire creates 1 Hash-typed Index on the implementing Session's `principalName`
+property, which corresponds to the Session attribute...
+
+----
+ org.springframework.session.FindByIndexNameSessionRepository.PRINCIPAL_NAME_INDEX_NAME
+----
+
+This enables developers using the `GemFireOperationsSessionRepository` programmatically to query and find all Sessions
+with a given principal name efficiently.
+
+Additionally, Spring Session Data GemFire will create a Range-based Index on the implementing Session's Map-type
+`attributes` property (i.e. on any arbitrary Session attribute) when a developer identifies 1 or more named Session
+attributes that should be indexed by GemFire.
+
+Sessions attributes to index can be specified with the `indexableSessionAttributes` attribute on the `@EnableGemFireHttpSession`
+annotation.  A developer adds this annotation to their Spring application `@Configuration` class when s/he wishes to
+enable Spring Session support for HttpSession backed by GemFire.
+
+For example:
+
+.ApplicationConfiguration.java
+----
+@EnableGemFireHttpSession(indexableSessionAttributes = { "name1", "name2", "name3" })
+class ApplicationConfiguration {
+
+  // @Bean definition methods implemented here
+}
+----
+
+NOTE: Only Session attribute names identified in the `@EnableGemFireHttpSession` annotation's `indexableSessionAttributes`
+attribute will have an Index defined.  All other Session attributes will not be indexed.
+
+However, there is one caveat. Any values stored in indexable Session attributes must implement the `java.lang.Comparable<T>`
+interface. If those object values do not implement `Comparable`, then GemFire will throw an error on startup when the
+Index is defined for Regions with persistent Session data, or when an attempt is made at runtime to assign the indexable
+Session attribute a value that is not `Comparable` and the Session is saved to GemFire.
+
+NOTE: Any Session attribute that is not indexed may store non-`Comparable` values.
+
+To learn more about GemFire's Range-based Indexes, see http://gemfire.docs.pivotal.io/docs-gemfire/latest/developing/query_index/creating_map_indexes.html[Creating Indexes on Map Fields].
+
+To learn more about GemFire Indexing in general, see http://gemfire.docs.pivotal.io/docs-gemfire/latest/developing/query_index/query_index.html[Working with Indexes].
+
 [[httpsession-how]]
 === How HttpSession Integration Works