Document properties migrator usage

Closes gh-13600

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

@@ -451,6 +451,26 @@ that provides detailed upgrade instructions. Check also the
 {github-wiki}["`release notes`"] for a list of "`new and noteworthy`" features for each
 release.
 
+When upgrading to a new feature release, some properties may have been renamed or removed.
+Spring Boot provides a way to analyze your application's environment and print diagnostics
+at startup, but also temporarily migrate properties at runtime for you. To enable that
+feature, add the following dependency to your project:
+
+[source,xml,indent=0]
+----
+	<dependency>
+		<groupId>org.springframework.boot</groupId>
+		<artifactId>spring-boot-properties-migrator</artifactId>
+		<scope>runtime</scope>
+	</dependency>
+----
+
+WARNING: properties that are added late to the environment, such as when using
+`@PropertySource`, will not be taken into account.
+
+NOTE: Once you're done with the migration, please make sure to remove this module from
+your project's dependencies.
+
 To upgrade an existing CLI installation, use the appropriate package manager command (for
 example, `brew upgrade`) or, if you manually installed the CLI, follow the
 <<getting-started-manual-cli-installation, standard instructions>>, remembering to update