Writing a readme on using the new build

Runtimes/Core/Readme.md

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+# SwiftCore
+
+SwiftCore contains core libraries that sit below the platform overlays.
+These include the standard library and associated runtimes, SwiftOnoneSupport,
+and the concurrency runtimes.
+
+## Build Instructions
+
+> [!IMPORTANT]
+> The standard library requires that it is built with a Swift compiler that is
+> at least as new as the standard library sources. You will likely need to
+> build the compiler as you would normally.
+> In these instructions, `<swiftc>` is the path to your just-built Swift
+> compiler.
+
+Run these commands from the `Runtimes/Core` directory.
+
+Build a standard library with the default configuration. This builds for the
+system that the command was run on, usually resulting in a static archive
+without optimizations applied.
+
+```sh
+cmake -B build -S . -G Ninja -DCMAKE_Swift_COMPILER=<swiftc>
+cmake --build build
+DESTDIR=/tmp/staging-dir cmake --install build --prefix /usr
+```
+
+The `DESTDIR` environment variable sets the staging location so that the result
+of the build isn't installed directly to `/usr` on the system performing the
+build.
+
+> [!NOTE]
+> The `DESTDIR` environment variable is not portable to Windows.
+> Use `CMAKE_INSTALL_PREFIX` or the `--prefix` flag to set the staging location.
+
+To build the runtimes as dynamic libraries, pass `-DBUILD_SHARED_LIBS=YES` to
+CMake.
+
+```sh
+cmake -B build -S . -G Ninja -DBUILD_SHARED_LIBS=YES -DCMAKE_Swift_COMPILER=<swiftc>
+cmake --build build
+```
+
+To enable semantic editing, ensure you're running CMake 3.29 or newer and enable
+`CMAKE_EXPORT_COMPILE_COMMANDS`.
+
+```sh
+cmake -B build -DCMAKE_EXPORT_COMPILE_COMMANDS=YES
+```
+
+Some editors will look under `<source-root>/build` for the generated
+`compile_commands.json` while others do not.
+On UNIX-like systems, run `ln -s build/compile_commands.json`.
+On Windows, creating symlinks requires administrator privileges, but can be done
+with CMake `cmake -E create_symlink build\compile_commands.json
+compile_commands.json`. If you do not have administrator privileges, you can
+copy the file from your build directory into the root directory of your sources.
+Note that the copied file won't be updated so you will need to copy the file
+each time you re-run CMake.
+
+The compile-commands are specific to the current build configuration, so the
+semantic results shown in the editor match what you are currently building.
+
+There are many more knobs for configuration. From the build directory, run
+`ccmake` to edit the build configuration to your liking.
+
+### Reproducing a Build
+
+The knobs make it easy to build the standard library to your needs, but can make
+it challenging to reproduce a specific build configuration.
+CMake has cache files to specify how to position the knobs for a given
+configuration.
+
+These caches live under `cmake/caches`, but can live anywhere.
+The following command uses the `x86_64-MacOSX.cmake` cache file to reproduce the
+Apple Intel macOS standard library build.
+
+
+```sh
+cmake -B build -S . -G Ninja \
+    -DCMAKE_OSX_DEPLOYMENT_TARGET=15.3 \
+    -DCMAKE_OSX_SYSROOT=macosx \
+    -DCMAKE_Swift_COMPILER=<swiftc> \
+    --toolchain cmake/caches/Vendors/Apple/Darwin.toolchain.cmake \
+    -C cmake/caches/Vendors/Apple/x86_64-MacOSX.cmake
+cmake --build build
+```
+
+> [!NOTE]
+> Variables are evaluated in order of appearance on the command line.
+> The `x86_64-MacOSX.cmake` cache requires that `CMAKE_OSX_DEPLOYMENT_TARGET` is
+> set before usage. Therefore, setting `CMAKE_OSX_DEPLOYMENT_TARGET` must come
+> before specifying the cache or you will get an error message.