JetBrains: Optional excludes for auto-import

Add extra commented section to use when using Gradle or Maven auto-import.
These are mentioned in the original reference for optional excludes (https://intellij-support.jetbrains.com/hc/en-us/articles/206544839).
If this would be better as a separate, non-commented ignore file, let me know and I will resubmit.

Global/JetBrains.gitignore

@@ -21,6 +21,14 @@
 .idea/**/gradle.xml
 .idea/**/libraries
 
+# Gradle and Maven with auto-import
+# When using Gradle or Maven with auto-import, you should exclude module files,
+# since they will be recreated, and may cause churn.  Uncomment if using
+# auto-import.
+# .idea/modules.xml
+# .idea/**/*.iml
+# .idea/libraries/**/*.xml
+
 # CMake
 cmake-build-*/