docs: add usage guide in readme (#46)

* docs(readme): add example usages section

* docs(readme): add integration section

Author: threeal

README.md

@@ -11,6 +11,56 @@ It also supports sparse checkout for checking out specific files from a large re
 - Checking out a Git repository on a specific reference.
 - Support for sparse checkout.
 
+## Integration
+
+This module can be integrated into a CMake project in the following ways:
+
+- Manually download the [`GitCheckout.cmake`](./cmake/GitCheckout.cmake) file and include it in the CMake project.
+- Use [`file(DOWNLOAD)`](https://cmake.org/cmake/help/latest/command/file.html#download) to automatically download the `GitCheckout.cmake` file.
+- Use [CPM.cmake](https://github.com/cpm-cmake/CPM.cmake) to add this package to the CMake project.
+
+## Example Usages
+
+This example demonstrates how to clone and check out a Git repository hosted on GitHub:
+
+```cmake
+git_checkout("https://github.com/user/project")
+```
+
+### Specify Output Directory
+
+Use the `DIRECTORY` option to specify the output directory for cloning the Git repository:
+
+```cmake
+git_checkout(
+  "https://github.com/user/project"
+  DIRECTORY path/to/output
+)
+```
+
+### Specify Checkout Reference
+
+By default, a Git repository will be checked out on the default branch. To check out on a specific commit hash, branch, or tag, use the `REF` option:
+
+```cmake
+git_checkout(
+  "https://github.com/user/project"
+  REF latest
+)
+```
+
+### Use Sparse Checkout
+
+To save bandwidth, it is recommended to use a sparse checkout to check out only specific files from the Git repository, especially on a large repository.
+To do this, use the `SPARSE_CHECKOUT` option to list patterns of files to be checked out sparsely:
+
+```cmake
+git_checkout(
+  "https://github.com/user/project"
+  SPARSE_CHECKOUT src test
+)
+```
+
 ## License
 
 This project is licensed under the terms of the [MIT License](./LICENSE).