Add some documentation about Java 6 compatibility

Document the need to use a different JTA 1.2 API jar and Tomcat 7 or
Jetty 8 if you’re running on Java 6.

Closes gh-2347

spring-boot-docs/src/main/asciidoc/getting-started.adoc

@@ -36,11 +36,12 @@ diverge from the defaults.
 
 [[getting-started-system-requirements]]
 == System Requirements
-Spring Boot {spring-boot-version} requires http://www.java.com[Java 6] and Spring
-Framework 4.1.3 or above. Explicit build support is provided for Maven (3.2+) and
-Gradle (1.12+).
+By default, Spring Boot {spring-boot-version} requires http://www.java.com[Java 7] and
+Spring Framework 4.1.3 or above. You can use Spring Boot with Java 6 with some additional
+configuration. See <<howto.adoc#howto-use-java-6>> for more details. Explicit build support
+is provided for Maven (3.2+) and Gradle (1.12+).
 
-TIP: Although you can use Spring Boot with Java 6, we generally recommend Java 8 if at
+TIP: Although you can use Spring Boot with Java 6 or 7, we generally recommend Java 8 if at
 all possible.
 
 === Servlet containers

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -2201,6 +2201,35 @@ after which you can run the application with
 
 
 
+[[howto-use-java-6]]
+=== How to use Java 6
+If you want to use Spring Boot with Java 6 there are a small number of configuration
+changes that you will have to make. The exact changes depend on your application's
+functionality.
+
+
+
+[[howto-use-java-6-embedded-container]]
+==== Embedded servlet container compatibility
+
+If you are using one of Boot's embedded Servlet containers you will have to use a
+Java 6-compatible container. Both Tomcat 7 and Jetty 8 are Java 6 compatible. See
+<<howto-use-tomcat-7>> and <<howto-use-jetty-8>> for details.
+
+
+
+[[how-to-use-java-6-jta-api]]
+==== JTA API compatibility
+
+While the Java Transaction API itself doesn't require Java 7 the official API jar
+contains classes that have been built to require Java 7. If you are using JTA then
+you will need to replace the official JTA 1.2 API jar with one that has been built
+to work on Java 6. To do so, exclude any transitive dependencies on
+`javax.transaction:javax.transaction-api` and replace them with a dependency on
+`org.jboss.spec.javax.transaction:jboss-transaction-api_1.2_spec:1.0.0.Final`
+
+
+
 [[howto-traditional-deployment]]
 == Traditional deployment