Create a README for build presets

tools/cmake/preset/README.md

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+# ExecuTorch Build Presets
+
+Build presets are a collection of cmake arguments that configure flavors of ExecuTorch libraries. Build presets are intended to help answer questions like:
+
+- "How do I build the ET library used in the pip wheel?" → use [pybind.cmake](./pybind.cmake)
+- "How do I build the iOS library?" → use [ios.cmake](./ios.cmake)
+- "How do I build what's required to run an LLM?" → use [llm.cmake](./llm.cmake)
+
+## Why?
+
+See: https://github.com/pytorch/executorch/discussions/10661. tl;dr instead of turning on multiple opaque/unknown flags, you can reduce it to:
+
+```bash
+$ cmake --preset macos
+$ cmake --build cmake-out -j100 --target executor_runner
+```
+
+## Working with Presets
+
+> [!TIP]
+> Configurable presets options and their default values are stored in [default.cmake](./default.cmake).
+
+### Within ExecuTorch
+
+If you're developing ExecuTorch directly, you can use the `cmake --preset` command:
+
+```bash
+# List all the presets buildable on your machine
+$ cmake --list-presets
+
+# Build a preset
+$ cmake --preset llm
+
+# Build a preset with one-off configuration change
+$ cmake -DEXECUTORCH_BUILD_MPS=OFF --preset llm
+```
+
+The cmake presets roughly map to the ExecuTorch presets and are explicitly listed in [CMakePresets.json](../../../CMakePresets.json). Note that you are encouraged to rely on presets when build locally and adding build/tests in CI — CI should do what a developer would do and nothing more!
+
+### Including ExecuTorch as Third-party Library
+
+#### Choose a built-in preset
+
+You can include ExecuTorch like any other cmake project in your project:
+```cmake
+add_subdirectory(executorch)
+```
+However, note that since a preset isn't specified, it will use the [default](./default.cmake) options. This likely not what you want — you likely want to use a preset. Check for presets available in [tools/cmake/preset](.) and explicitly choose which preset to use:
+
+```cmake
+set(EXECUTORCH_BUILD_PRESET_FILE executorch/tools/cmake/preset/llm.cmake)
+add_subdirectory(executorch)
+```
+
+Even when using a preset, you can explicitly override specific preset configurations:
+
+```cmake
+set(EXECUTORCH_BUILD_PRESET_FILE executorch/tools/cmake/preset/llm.cmake)
+
+# Although llm.cmake might have turned on `EXECUTORCH_BUILD_MPS`, you can turn if off
+set(EXECUTORCH_BUILD_MPS OFF)
+
+add_subdirectory(executorch)
+```
+
+#### Creating your own preset
+
+Preset files are just cmake files that turn on/off ExecuTorch configs. For example if we want to build the runner with a CoreML backend, create your cmake preset file (i.e. `my_projects_custom_preset.cmake`):
+
+```cmake
+# File: my_projects_custom_preset.cmake
+
+set_overridable_option(EXECUTORCH_BUILD_COREML ON)
+set_overridable_option(EXECUTORCH_BUILD_EXECUTOR_RUNNER ON)
+```
+
+We use `set_overridable_option` so that it allows the option to be overriden via the command line using `-D`. Feel free to use the standard `set(...)` function to prevent overriding options.
+
+Once the preset is created, set `EXECUTORCH_BUILD_PRESET_FILE` before adding ExecuTorch:
+
+```cmake
+set(EXECUTORCH_BUILD_PRESET_FILE my_projects_custom_preset.cmake)
+add_subdirectory(executorch)
+```