update debugging section in the README (#1451)

Author: matthewbastien

README.md

@@ -101,10 +101,12 @@ Additionally, the extension will monitor `Package.swift` and `Package.resolved`
 
 The Swift extension uses the [LLDB DAP](https://marketplace.visualstudio.com/items?itemName=llvm-vs-code-extensions.lldb-dap) extension for debugging.
 
-When you open a Swift package (a directory containing a `Package.swift` file), the extension automatically generates build tasks and launch configurations for each executable within the package. Additionally, if the package includes tests, the extension creates a configuration specifically designed to run those tests. These configurations all leverage the CodeLLDB extension as the debugger of choice.
+When you open a Swift package (a directory containing a `Package.swift` file), the extension automatically generates build tasks and launch configurations for each executable within the package. Additionally, if the package includes tests, the extension creates a configuration specifically designed to run those tests. These configurations all leverage the LLDB DAP extension as the debugger of choice.
 
 Use the **Run > Start Debugging** menu item to run an executable and start debugging. If you have multiple launch configurations you can choose which launch configuration to use in the debugger view.
 
+LLDB DAP is only available starting in Swift 6.0. On older versions of Swift the [CodeLLDB](https://marketplace.visualstudio.com/items?itemName=vadimcn.vscode-lldb) extension will be used for debugging instead. You will be prompted to install the CodeLLDB extension in this case.
+
 CodeLLDB includes a version of `lldb` that it uses by default for debugging, but this version of `lldb` doesn’t support Swift. The Swift extension will automatically identify the required version and offer to update the CodeLLDB configuration as necessary so that debugging is supported.
 
 ### Test Explorer