chore: update README with note about hierarchical project support in IntelliJ (#698)

Author: ianbotsf

README.md

@@ -132,6 +132,24 @@ aws.services=-location,-dynamodb
 # Generate all services except those using the restJson1 protocol:
 aws.protocols=-restJson1
 ```
+
+### IntelliJ multiplatform compatibility
+
+Due to an outstanding issue in IntelliJ ([KTIJ-21583](https://youtrack.jetbrains.com/issue/KTIJ-21583)), the IDE will
+sometimes fail to resolve symbols in composite projects like
+[**smithy-kotlin**](https://github.com/awslabs/smithy-kotlin) and
+[**aws-crt-kotlin**](https://github.com/awslabs/aws-crt-kotlin). When using either of these projects in a composite
+build in IntelliJ, you may need to disable hierarchical project support. Add (or modify) a **local.properties** file in
+**smithy-kotlin** and **aws-crt-kotlin** with the following contents:
+
+```
+# FIXME - see https://youtrack.jetbrains.com/issue/KTIJ-21583
+kotlin.mpp.hierarchicalStructureSupport=false
+```
+
+**Note**: These files should be created in the **smithy-kotlin** and **aws-crt-kotlin** projects, not the
+**aws-sdk-kotlin** project.
+
 ### Debugging
 
 See [Debugging](docs/debugging.md)