[lldb] Show --lldb-use-system-debugserver in readme (#3957)

* [lldb] Show --lldb-use-system-debugserver in readme

* Document macos-setup-codesign.sh

* Improved wording of note

(cherry picked from commit 0a2f1ba)

Author: kastiglione

lldb/README.md

@@ -28,9 +28,13 @@ mkdir myswift
 cd myswift
 git clone https://github.com/apple/swift.git swift
 ./swift/utils/update-checkout
+./llvm-project/lldb/scripts/macos-setup-codesign.sh
 ./swift/utils/build-script -r --lldb
 ```
 
+Note: The codesigning setup script needs to be run only once. It's not required
+if `build-script` is run with the `--lldb-use-system-debugserver` flag.
+
 # Contribution Subtleties
 
 The swift-lldb project enhances the core LLDB project developed under