Added TROUBLESHOOTING.md

Author: fwcd

TROUBLESHOOTING.md

@@ -0,0 +1,17 @@
+# Troubleshooting
+
+## The tests fail with java.lang.NoSuchMethodError
+* After updating the Kotlin version, there may be multiple copies of the compiler plugin in `lib-kotlin`, for example:
+
+```
+lib-kotlin
+├───j2k-1.2.72-release-68.jar <- old version
+├───j2k-1.3.11-release-272.jar
+├───kotlin-plugin-1.2.72-release-68.jar <- old version
+└───kotlin-plugin-1.3.11-release-272.jar
+```
+
+* The issue is that the compiler finds multiple versions of the same class on the classpath
+* To fix this, simply remove the older JARs
+* If that still does not work, delete the entire `lib-kotlin` folder
+    * Gradle will automatically re-download the necessary files once the project is built again