Note different document formats in reference guide

Fixes gh-570

Author: Phillip Webb

spring-boot-docs/src/main/asciidoc/documentation-overview.adoc

@@ -6,10 +6,24 @@
 This section provides a brief overview of Spring Boot reference documentation. Think of
 it as map for the rest of the document. You can read this reference guide in a linear
 fashion, or you can skip sections if something doesn't interest you.
-
 --
 
 
+
+[[boot-documentation-about]]
+== About the documentation
+The Spring Boot reference guide is available as {spring-boot-docs}/html[html],
+{spring-boot-docs}/pdf/spring-boot-reference.pdf[pdf]
+and {spring-boot-docs}/epub/spring-boot-reference.epub[epub] documents. The latest copy
+is available at {spring-boot-docs-current}.
+
+Copies of this document may be made for your own use and for
+distribution to others, provided that you do not charge any fee for such copies and
+further provided that each copy contains this Copyright Notice, whether distributed in
+print or electronically.
+
+
+
 [[boot-documentation-getting-help]]
 == Getting help
 Having trouble with Spring Boot, We'd like to help!

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

@@ -9,6 +9,8 @@ Phillip Webb; Dave Syer; Josh Long; Stéphane Nicoll; Rob Winch;
 :spring-boot-repo: snapshot
 :github-tag: master
 :spring-boot-docs-version: current
+:spring-boot-docs: http://docs.spring.io/spring-boot/docs/{spring-boot-docs-version}/reference
+:spring-boot-docs-current: http://docs.spring.io/spring-boot/docs/current/reference
 :github-repo: spring-projects/spring-boot
 :github-raw: http://raw.github.com/{github-repo}/{github-tag}
 :github-code: http://github.com/{github-repo}/tree/{github-tag}