Fix broken links in reference appendices

Author: garyrussell

src/reference/asciidoc/changes-1.0-2.0.adoc

@@ -3,7 +3,7 @@
 
 For a detailed migration guide in regards to upgrading an existing application that uses Spring Integration older than version 2.0, please see:
 
-null
+https://github.com/spring-projects/spring-integration/wiki/Spring-Integration-1.0-to-2.0-Migration-Guide[Spring Integration 1.0 to 2.0 Migration Guide]
 
 [[migration-spring-30-support]]
 ==== Spring 3 support

src/reference/asciidoc/changes-2.0-2.1.adoc

@@ -195,20 +195,20 @@ The old 1.0 and 2.0 schemas are still there, but if an Application Context still
 
 Since version 2.0, the Spring Integration project uses http://git-scm.com/[Git] for version control.
 In order to increase community visibility even further, the project was moved from SpringSource hosted Git repositories to http://www.github.com/[Github].
-The Spring Integration Git repository is located at:null
+The Spring Integration Git repository is located at: https://github.com/spring-projects/spring-integration[spring-integration].
 
 For the project we also improved the process of providing code contributions and we ensure that every commit is peer-reviewed.
 In fact, core committers now follow the same process as contributors.
 For more details please see:
 
-null
+https://github.com/spring-projects/spring-integration/blob/master/CONTRIBUTING.adoc[Contributing].
 
 [[x2.1-sonar]]
 ===== Improved Source Code Visibility with Sonar
 
-In an effort to provide better source code visibility and consequently to monitor the quality of Spring Integration's source code, an instance of http://www.sonarsource.org/[Sonar] was setup and metrics are gathered nightly and made avaiblable at:
+In an effort to provide better source code visibility and consequently to monitor the quality of Spring Integration's source code, an instance of http://www.sonarsource.org/[Sonar] was setup and metrics are gathered nightly and made available at:
 
-null
+https://sonar.spring.io/dashboard?id=org.springframework.integration%3Aspring-integration%3Amaster[sonar.spring.io].
 
 [[x2.1-new-samples]]
 ==== New Samples
@@ -217,4 +217,4 @@ For the 2.1 release of Spring Integration we also expanded the Spring Integratio
 samples covering AMQP support, the new payload enricher, a sample illustrating techniques for testing Spring Integration flow fragments, as well as an example for executing Stored Procedures against Oracle.
 For details please visit:
 
-null
+https://github.com/spring-projects/spring-integration-samples[spring-integration-samples].