Add website (#65)

website/docs/guide/installation.md

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+# Installation
+
+## Check if your device is supported
+
+Download the KernelSU manager app from [github releases](https://github.com/tiann/KernelSU/releases) or [github actions](https://github.com/tiann/KernelSU/actions/workflows/build-manager.yml), and then install the app to device and open the app:
+
+- If the app shows `Unsupported`, it means KernelSU is not supported for your device.
+- If the app shows `Not installed`, then your devices is supported by KernelSU.
+
+## Find proper boot.img
+
+KernelSU provides a general boot.img for GKI devices, you should flash the boot.img to the boot partition of your device.
+
+You can download the boot.img from [github actions for kernel](https://github.com/tiann/KernelSU/actions/workflows/build-kernel.yml), Please be aware that your should use the right version of boot.img. For example, if your device show that the kernel is `5.10.101`, then you need to download the `5.10.101-xxxx.boot.xxx`.
+
+And also, please check your stock boot.img's format, you should use the right format, such as `lz4`、`gz`.
+
+## Flash the boot.img to device
+
+Connect your device with `adb` and then execute `adb reboot bootloader` to enter fastboot mode, and then use this command to flash KernelSU:
+
+```sh
+fastboot flash boot boot.img
+```
+
+## Reboot
+
+When flashed, you should reboot your device:
+
+```sh
+fastboot reboot
+```