Update README to reflect the build now using Gradle

Closes gh-19617

README.adoc

@@ -93,63 +93,29 @@ requests. If you want to raise an issue, please follow the recommendations below
 == Building from Source
 You don't need to build from source to use Spring Boot (binaries in
 https://repo.spring.io[repo.spring.io]), but if you want to try out the latest and
-greatest, Spring Boot can be easily built with the
-https://github.com/takari/maven-wrapper[maven wrapper]. You also need JDK 1.8.
+greatest, Spring Boot can be built and published to your local Maven cache using the
+https://docs.gradle.org/current/userguide/gradle_wrapper.html[Gradle wrapper]. You also
+need JDK 1.8.
 
 [indent=0]
 ----
-	$ ./mvnw clean install
+	$ ./gradlew publishToMavenLocal
 ----
 
-If you want to build with the regular `mvn` command, you will need
-https://maven.apache.org/run-maven/index.html[Maven v3.5.0 or above].
+This will build all of the jars and documentation and publish every to your local
+Maven cache. It won't run any of the tests. If you want to build everything, use the
+`build` task:
 
-NOTE: You may need to increase the amount of memory available to Maven by setting
-a `MAVEN_OPTS` environment variable with the value `-Xmx512m`. Remember
-to set the corresponding property in your IDE as well if you are building and running
-tests there (e.g. in Eclipse go to `Preferences->Java->Installed JREs` and edit the
-JRE definition so that all processes are launched with those arguments). This property
-is automatically set if you use the maven wrapper.
+[indent=0]
+----
+	$ ./gradlew build
+----
 
 _Also see link:CONTRIBUTING.adoc[CONTRIBUTING.adoc] if you wish to submit pull requests,
 and in particular please fill out the
 https://support.springsource.com/spring_committer_signup[Contributor's Agreement]
 before your first change, however trivial._
 
-=== Building reference documentation
-
-First of all, make sure you have built the project:
-
-[indent=0]
-----
-	$ ./mvnw clean install
-----
-
-The reference documentation requires the documentation of the Maven plugin to be
-available so you need to build that first since it's not generated by default.
-
-[indent=0]
-----
-	$ ./mvnw clean install -pl spring-boot-project/spring-boot-tools/spring-boot-maven-plugin -Pdefault,full
-----
-
-The documentation also includes auto-generated information about the starters. You might
-have that in your local repository already (per the first step) but if you want to refresh
-it:
-
-[indent=0]
-----
-	$ ./mvnw clean install -f spring-boot-project/spring-boot-starters
-----
-
-Once this is done, you can build the reference documentation with the command below:
-
-[indent=0]
-----
-	$ ./mvnw clean prepare-package -pl spring-boot-project/spring-boot-docs -Pdefault,full
-----
-
-TIP: The generated documentation is available from `spring-boot-project/spring-boot-docs/target/generated-docs/reference/html`
 
 
 == Modules