add notes about building Release

Tools/llm-tool/README.md

@@ -43,3 +43,18 @@ The easiest way to do this is drag the Products/llm-tool into Terminal to get th
 DYLD_FRAMEWORK_PATH=~/Library/Developer/Xcode/DerivedData/mlx-examples-swift-ceuohnhzsownvsbbleukxoksddja/Build/Products/Debug ~/Library/Developer/Xcode/DerivedData/mlx-examples-swift-ceuohnhzsownvsbbleukxoksddja/Build/Products/Debug/llm-tool --prompt "swift programming language"
 ```
 
+### Troubleshooting
+
+If the program crashes with a very deep stack trace you may need to build
+in Release configuration.  This seems to depend on the size of the model.
+
+There are a couple options:
+
+- build Release
+- force the model evaluation to run on the main thread, e.g. using @MainActor
+- build `Cmlx` with optimizations by modifying `mlx/Package.swift` and adding `.unsafeOptions(["-O3"]),` around line 87
+
+Building in Release / optimizations will remove a lot of tail calls in the C++ 
+layer.  These lead to the stack overflows.
+
+See discussion here: https://github.com/ml-explore/mlx-swift-examples/issues/3