Document IREE-specific CMake options

First attempt to solve #2111. There is still room to improve the layout.

Closes https://github.com/google/iree/pull/2230

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/2230 from iml130:docs b607d5e17b0375edb5249480b8197df15bf8c77c
PiperOrigin-RevId: 317200804

docs/GetStarted/cmake_options_and_variables.md

diff --git a/docs/GetStarted/cmake_options_and_variables.md b/docs/GetStarted/cmake_options_and_variables.md
new file mode 100644
index 0000000..899fa19
--- /dev/null
+++ b/docs/GetStarted/cmake_options_and_variables.md
@@ -0,0 +1,74 @@
+# IREE-specific CMake Options and Variables
+
+This gives a brief explanation of IREE specific CMake options and variables.
+
+*   `IREE_ENABLE_RUNTIME_TRACING`:BOOL
+
+    Enables instrumented runtime tracing. Defaults to `OFF`.
+
+*   `IREE_ENABLE_MLIR`:BOOL
+
+    Enables MLIR/LLVM dependencies. Defaults to `ON`.
+
+*   `IREE_ENABLE_EMITC`:BOOL
+
+    Enables MLIR EmitC dependencies. Defaults to OFF. Requires that
+    `IREE_ENABLE_MLIR` is set to `ON`.
+
+*   `IREE_BUILD_COMPILER`:BOOL
+
+    Builds the IREE compiler. Defaults to `ON`.
+
+*   `IREE_BUILD_TESTS`:BOOL
+
+    Builds IREE unit tests. Defaults to `ON`.
+
+*   `IREE_BUILD_DOCS`:BOOL
+
+    Builds IREE documentation. Defaults to `OFF`.
+
+*   `IREE_BUILD_SAMPLES`:BOOL
+
+    Builds IREE sample projects. Defaults to `ON`.
+
+*   `IREE_BUILD_DEBUGGER`:BOOL
+
+    Builds the IREE debugger app. Defaults to `OFF`.
+
+*   `IREE_BUILD_PYTHON_BINDINGS`:BOOL
+
+    Builds the IREE python bindings Defaults to `OFF`.
+
+*   `IREE_BUILD_EXPERIMENTAL`:BOOL
+
+    Builds experimental projects. Defaults to `OFF`.
+
+*   `IREE_HAL_DRIVERS_TO_BUILD`:STRING
+
+    *This does not have any effect at the moment, but will be supported in the
+    future!* Semicolon-separated list of HAL drivers to build, or `all` for
+    building all HAL drivers. Case-insensitive. Defaults to `all`. Example:
+    `-DIREE_HAL_DRIVERS_TO_BUILD="Vulkan;VMLA"`.
+
+*   `IREE_TARGET_BACKENDS_TO_BUILD`:STRING
+
+    *This does not have any effect at the moment, but will be supported in the
+    future!* Semicolon-separated list of HAL drivers to build, or `all` for
+    building all HAL drivers. Case-insensitive. Defaults to `all`. Example:
+    `-DIREE_HAL_DRIVERS_TO_BUILD="Vulkan_SPIRV;VMLA"`.
+
+*   `IREE_ENABLE_LLD`:BOOL
+
+    Use lld when linking. Defaults to `OFF`.
+
+*   `IREE_MLIR_DEP_MODE`:STRING
+
+    Defines the MLIR dependency mode. Case-sensitive. Can be `BUNDLED`,
+    `DISABLED` or `INSTALLED`. Defaults to `BUNDLED`. If set to `INSTALLED`, the
+    variable `MLIR_DIR` needs to be passed and that LLVM needs to be compiled
+    with `LLVM_ENABLE_RTTI` set to `ON`.
+
+*   `MLIR_DIR`:STRING
+
+    Specifies the path where to look for the installed MLIR/LLVM packages.
+    Required if `IREE_MLIR_DEP_MODE` is set to `INSTALLED`.