docs: add README.md

README.md

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+# InfiniRT
+
+InfiniRT is the runtime library of the InfiniCore project. It provides a stable runtime API for device management and memory operations across supported backends such as CPU and NVIDIA.
+
+The recommended public entry is:
+
+```cpp
+#include <infini/rt.h>
+```
+
+## Build and Install
+
+Configure, build, and install InfiniRT with CMake:
+
+```bash
+cmake -S . -B build -DCMAKE_INSTALL_PREFIX=/path/to/infini-rt-prefix
+cmake --build build -j
+cmake --install build
+```
+
+`infini-rt-prefix` means the installation prefix used by CMake. After installation, headers and libraries are placed under this prefix, for example:
+
+```text
+/path/to/infini-rt-prefix/include
+/path/to/infini-rt-prefix/lib
+```
+
+## Build Options
+
+Common options include:
+
+```bash
+-DWITH_CPU=ON
+-DWITH_NVIDIA=ON
+-DINFINI_RT_BUILD_TESTING=ON
+```
+
+Example CPU build:
+
+```bash
+cmake -S . -B build \
+  -DCMAKE_INSTALL_PREFIX=/path/to/infini-rt-prefix \
+  -DWITH_CPU=ON \
+  -DINFINI_RT_BUILD_TESTING=ON
+cmake --build build -j
+ctest --test-dir build --output-on-failure
+cmake --install build
+```
+
+Example NVIDIA build:
+
+```bash
+cmake -S . -B build \
+  -DCMAKE_INSTALL_PREFIX=/path/to/infini-rt-prefix \
+  -DWITH_NVIDIA=ON \
+  -DINFINI_RT_BUILD_TESTING=ON
+cmake --build build -j
+ctest --test-dir build --output-on-failure
+cmake --install build
+```
+
+## Basic Usage
+
+```cpp
+#include <cstddef>
+#include <infini/rt.h>
+
+int main() {
+  infini::rt::SetDevice({infini::rt::Device::Type::kCpu, 0});
+
+  constexpr std::size_t size = 1024;
+  void* ptr = nullptr;
+
+  infini::rt::Malloc(&ptr, size);
+  infini::rt::Memset(ptr, 0, size);
+  infini::rt::Free(ptr);
+
+  return 0;
+}
+```
+
+For NVIDIA:
+
+```cpp
+infini::rt::SetDevice({infini::rt::Device::Type::kNvidia, 0});
+```
+
+## Using Installed InfiniRT From Another Project
+
+Downstream projects should consume the installed headers and libraries instead of depending on the InfiniRT source tree.
+
+Typical include and library paths are:
+
+```text
+/path/to/infini-rt-prefix/include
+/path/to/infini-rt-prefix/lib
+```
+
+A simple compile command may look like:
+
+```bash
+c++ main.cc \
+  -I/path/to/infini-rt-prefix/include \
+  -L/path/to/infini-rt-prefix/lib \
+  -linfinirt
+```
+
+At runtime, make sure the dynamic linker can find `libinfinirt.so`, for example:
+
+```bash
+export LD_LIBRARY_PATH=/path/to/infini-rt-prefix/lib:$LD_LIBRARY_PATH
+```
+
+## Tests
+
+Enable C++ tests with:
+
+```bash
+-DINFINI_RT_BUILD_TESTING=ON
+```
+
+Then run:
+
+```bash
+ctest --test-dir build --output-on-failure
+```
+
+## Formatting
+
+C++ code should be formatted with `clang-format`.
+
+Python scripts should be checked and formatted with Ruff:
+
+```bash
+ruff format --check .
+ruff check .
+```
+
+## Contributing
+
+Please follow the repository's `CONTRIBUTING.md`.