增加 安装系统.

Signed-off-by: lion.chan <cy187lion@sina.com>

Software/System/RaspberryPi/Raspberry_Pi_4B_安装_Ubuntu_Server_20.04.md

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+# Raspberry Pi 4B 安装 Ubuntu Server 20.04
+
+## 1.述语
+
+* 宿主机：
+
+## 2.Raspberry Pi 4B 硬件配置
+
+## 3.安装系统前的准备
+
+首先宿主机需要安装 Raspberry Pi Imager 工具。
+
+```bash
+# Ubuntu
+sudo snap install rpi-imager
+# Manjaro
+sudo pacman -S rpi-imager
+```
+
+## 4.烧录系统
+
+![Operating System](./img/Raspberry_Pi_4B_安装_Ubuntu_Server_20.04/001.png)
+
+![Use custom](./img/Raspberry_Pi_4B_安装_Ubuntu_Server_20.04/002.png)
+
+![Storage](./img/Raspberry_Pi_4B_安装_Ubuntu_Server_20.04/003.png)
+
+![选择 TF 卡](./img/Raspberry_Pi_4B_安装_Ubuntu_Server_20.04/004.png)
+
+![WRITE](./img/Raspberry_Pi_4B_安装_Ubuntu_Server_20.04/005.png)
+
+![确认警告](./img/Raspberry_Pi_4B_安装_Ubuntu_Server_20.04/006.png)
+
+![写入过程](./img/Raspberry_Pi_4B_安装_Ubuntu_Server_20.04/007.png)
+
+## 外部参考资料
+
+1. [系统下载地址](https://cdimage.ubuntu.com/releases/20.04.2/release/ubuntu-20.04.2-preinstalled-server-arm64+raspi.img.xz)
+2. [How to install Ubuntu Desktop on Raspberry Pi 4](https://ubuntu.com/tutorials/how-to-install-ubuntu-desktop-on-raspberry-pi-4#1-overview)