Merge pull request #341 from cltnschlosser/cs_testingWithToolchains

Add section to README about testing with toolchains

Author: cltnschlosser

README.md

@@ -102,6 +102,13 @@ apple/swift-driver#208
 ```
 @swift-ci cross-repository testing facilities are described [here](https://github.com/apple/swift/blob/main/docs/ContinuousIntegration.md#cross-repository-testing).
 
+### Testing in Xcode with custom toolchain
+After the toolchain is installed, Xcode needs to be told to use it. This can mean two things, building the driver with the toolchain and telling the driver to use the toolchain when running.
+
+Building with the toolchain is easy, set the toolchain in Xcode: Menu Bar > Xcode > Toolchains > select your toolchain
+
+Running the driver requires setting the TOOLCHAINS environment variable. This tells xcrun which toolchain to use (on darwin xcrun is used to find tools). This variable is the name of the toolchain and not the path (ex: `Swift Development Snapshot`). Important note: xcrun lookup is lower priority than the SWIFT_EXEC_*_EXEC family of environment variables, the tools directory, and any tools in the same directory as the driver (This includes a driver installed in a toolchain). Even though TOOLCHAINS is not highest priority it's a convenient way to run the xctest suite using a custom toolchain.
+
 #### Preparing a Linux docker for debug
 
 When developing on macOS without quick access to a Linux machine, using a Linux Docker is often helpful when debugging.