Add documentation for Servlet 2.5 and GAE

Fixes gh-1104

spring-boot-docs/src/main/asciidoc/cloud-deployment.adoc

@@ -299,6 +299,10 @@ run the app. A http://issues.gradle.org/browse/GRADLE-2871[bug in Gradle] curren
 prevents you from using Gradle newer than 1.6.
 
 
+[[cloud-deployment-gae]]
+== Google App Engine
+Google App Engine is tied to the Servlet 2.5 API, so you can't deploy a Spring Application
+there without some modifications. See the <<howto.adoc#howto-servlet-2-5, Servlet 2.5 section>> of this guide.
 
 [[cloud-deployment-whats-next]]
 == What to read next

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

@@ -2064,3 +2064,7 @@ context.
 
 Applications that are not already Spring applications might be convertible to a Spring
 Boot application, and the guidance above might help, but your mileage may vary.
+
+[[howto-servlet-2-5]]
+===  Deploying a WAR in an Old (Servlet 2.5) Container
+include::servlet-2.5.adoc[]

spring-boot-docs/src/main/asciidoc/servlet-2.5.adoc

@@ -0,0 +1,61 @@
+Spring Boot uses Servet 3.0 APIs to initialize the `ServletContext`
+(register `Servlets` etc.)  so you can't use the same application out
+of the box in a Servlet 2.5 container. It *is* however possible to run
+a Spring Boot application on an older container with some special
+tools. If you include `org.springframework.boot:spring-boot-legacy` as
+a dependency
+(https://github.com/scratches/spring-boot-legacy[maintained
+separately] to the core of Spring Boot and currently available at
+1.0.0.RELEASE), all you should need to do is create a `web.xml` and
+declare a context listener to create the application context and your
+filters and servlets. The context listener is a special purpose one
+for Spring Boot, but the rest of it is normal for a Spring application
+in Servlet 2.5. Example:
+
+[source,xml,indent=0]
+----
+<?xml version="1.0" encoding="UTF-8"?>
+<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
+
+	<context-param>
+		<param-name>contextConfigLocation</param-name>
+		<param-value>demo.Application</param-value>
+	</context-param>
+
+	<listener>
+		<listener-class>org.springframework.boot.legacy.context.web.SpringBootContextLoaderListener</listener-class>
+	</listener>
+
+	<filter>
+		<filter-name>metricFilter</filter-name>
+		<filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
+	</filte>r
+
+	<filter-mapping>
+		<filter-name>metricFilter</filter-name>
+		<url-pattern>/*</url-pattern>
+	</filter-mapping>
+
+	<servlet>
+		<servlet-name>appServlet</servlet-name>
+		<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
+		<init-param>
+			<param-name>contextAttribute</param-name>
+			<param-value>org.springframework.web.context.WebApplicationContext.ROOT</param-value>
+		</init-param>
+		<load-on-startup>1</load-on-startup>
+	</servlet>
+
+	<servlet-mapping>
+		<servlet-name>appServlet</servlet-name>
+		<url-pattern>/</url-pattern>
+	</servlet-mapping>
+
+</web-app>
+----
+
+In this example we are using a single application context (the one created by the context listener) and 
+attaching it to the `DispatcherServlet` using an init parameter. This is normal in a Spring Boot
+application (you normally only have one application conext).