Add link to the samples and mention JUnit version

Fixes #297

Author: melix

docs/src/docs/asciidoc/gradle-plugin.adoc

@@ -12,6 +12,11 @@ For upgrading please take a look at the <<index.adoc#changelog,Changelog>>.
 
 == Quickstart
 
+[NOTE]
+====
+You can find full samples in https://github.com/graalvm/native-build-tools/tree/master/samples[the source repository].
+====
+
 === Adding the plugin
 
 Add following to `plugins` section of your project's `build.gradle` / `build.gradle.kts`:
@@ -215,6 +220,7 @@ When the `classpathJar` property is set, the `classpath` property is _ignored_.
 This plugin supports running tests on the
 https://junit.org/junit5/docs/current/user-guide/[JUnit Platform] as native images. This
 means that tests will be compiled and executed as native code.
+The minimal supported version is JUnit Platform 5.8.1.
 
 In theory, any `TestEngine` supported on the JUnit Platform should be supported by this
 plugin as long as the programming language used by the `TestEngine` and the programming

docs/src/docs/asciidoc/maven-plugin.adoc

@@ -14,6 +14,11 @@ For upgrading please take a look at the <<index.adoc#changelog,Changelog>>.
 [[quickstart]]
 == Quickstart
 
+[NOTE]
+====
+You can find full samples in https://github.com/graalvm/native-build-tools/tree/master/samples[the source repository].
+====
+
 This plugin first requires that you <<graalvm-setup.adoc#,setup GraalVM and `native-image` properly>>.
 It will then make use of Maven profiles to enable building and testing of native images.
 
@@ -296,6 +301,7 @@ In this case, the arguments that will be passed to the `native-image` executable
 This plugin supports running tests on the
 https://junit.org/junit5/docs/current/user-guide/[JUnit Platform] as native images. This
 means that tests will be compiled and executed as native code.
+The minimal supported version is JUnit Platform 5.8.1.
 
 In theory, any `TestEngine` supported on the JUnit Platform should be supported by this
 plugin as long as the programming language used by the `TestEngine` and the programming