Add docs for debugging swift-build (#3136)

This should aid others looking to finding a way to use swift-build with
rules_xcodeproj

Signed-off-by: Maxwell Elliott <maxwell@elliott.now>

Author: maxwellE

docs/swift-build/debugging-swift-build-locally.md

@@ -0,0 +1,31 @@
+# Debugging Swift Build Locally
+
+During the investigation into supporting Build With Proxy (BwP) for rules_xcodeproj
+there will be a need to debug [swift-build](https://github.com/swiftlang/swift-build) locally.
+Below are steps to help assist developers when trying to debug swift-build.
+
+## Setup
+
+First create a test project or find an existing project you wish to test with, copy the path
+to its xcodeproj file, we will refer to this as `LOCAL_TEST_XCODEPROJ_PATH`.
+
+```terminal
+git clone git@github.com:swiftlang/swift-build.git
+cd swift-build
+open Package.swift
+```
+
+Once the project is opened select the `swbuild` scheme and then select `Edit Scheme`
+
+Set `Arguments Passed on Launch` like so:
+
+* `build`
+* `LOCAL_TEST_XCODEPROJ_PATH` (using the real path of course)
+
+Then under `Environment Variables` set the following:
+
+* `XCBUILD_LAUNCH_IN_PROCESS=1`
+
+## Testing
+
+Now that that is all set up you can run the `swbuild` executable from Xcode and add breakpoints to the project. When you run this binary it is the same as if you built it using swift-build's `launch-xcode` plugin but it is in the current Xcode project.