Added documentation for Java 17 to site (#497)

For dealing with illegal access error.

src/site/apt/usage.apt.vm

@@ -34,6 +34,26 @@ Usage
 
   Some brief examples on how to use this plugin.
 
+* Using Java 17 and later
+
+  Java 17 is the first LTS release to enforce strong encapsulation. For
+  <<<ktlint>>> to work we need to add
+  <<<--add-opens java.base/java.lang=ALL-UNNAMED>>> to the JVM arguments.
+
+  We recommend that you add a <<<.mvn/jvm.config>>> file (relative to the top
+  level project directory) to all of your projects using this plugin. The file
+  should have the following contents:
+
++----------+
+--add-opens java.base/java.lang=ALL-UNNAMED
++----------+
+
+  We also recommend adding this to all of your projects using this plugin and
+  building with Java 11, as it'll suppress an illegal-access warning during the
+  build.
+
+  For other options see: {{https://maven.apache.org/configure.html}}
+
 * Formatting and checking for violations as part of the build
 
   Probably the most common use case is to want to both format and check your