List Gradle's Shadow Plugin as an alternative to Boot's loader

Closes spring-projectsgh-1828

Author: wilkinsona

spring-boot-project/spring-boot-docs/src/main/asciidoc/appendix-executable-jar-format.adoc

@@ -331,4 +331,5 @@ following alternatives:
 * https://maven.apache.org/plugins/maven-shade-plugin/[Maven Shade Plugin]
 * http://www.jdotsoft.com/JarClassLoader.php[JarClassLoader]
 * http://one-jar.sourceforge.net[OneJar]
+* https://imperceptiblethoughts.com/shadow/[Gradle Shadow Plugin]