Add README section about JDK 16

Author: tschuchortdev

README.md

@@ -163,6 +163,32 @@ All code generated by the KSP processor will be written into the `KotlinCompilat
 - [Guardsquare/proguard](https://github.com/Guardsquare/proguard)
 - your project...
 
+## Java 16 compatibility
+
+With the release of Java 16 the access control of the new Jigsaw module system is starting to be enforced by the JVM. Unfortunately, this impacts kotlin-compile-testing because KAPT still tries to access classes of javac that are not exported by the jdk.compiler module, leading to errors such as:
+```
+java.lang.IllegalAccessError: class org.jetbrains.kotlin.kapt3.base.KaptContext (in unnamed module @0x43b6aa9d) cannot access class com.sun.tools.javac.util.Context (in module jdk.compiler) because module jdk.compiler does not export com.sun.tools.javac.util to unnamed module @0x43b6aa9d
+```
+To mitigate this problem, you have to add the following code to your module's `build.gradle` file:
+```
+if (JavaVersion.current() >= JavaVersion.VERSION_16) {
+    test {
+        jvmArgs = [
+            "--illegal-access=permit",
+            "--add-opens=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED",
+            "--add-opens=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED",
+            "--add-opens=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED",
+            "--add-opens=jdk.compiler/com.sun.tools.javac.jvm=ALL-UNNAMED",
+            "--add-opens=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED",
+            "--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED",
+            "--add-opens=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED",
+            "--add-opens=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED"
+        ]
+    }
+}
+```
+Since the kotlin compilation tests run in the same process as the test runner, these options have to be added manually and can not be set automatically by the kotlin-compile-testing library.
+
 ## License
 
 Copyright (C) 2019 Thilo Schuchort