Add firmware download instructions

Author: ophub

build-armbian/README.cn.md

@@ -8,6 +8,8 @@
 
 这里存放的文件是打包 Armbian 时需要使用的相关文件。其中 `common-files` 目录下的是通用文件，`platform-files` 目录下是各平台的差异化文件。
 
+需要的固件将从 [ophub/firmware](https://github.com/ophub/firmware) 仓库自动下载至 `common-files/usr/lib/firmware` 目录。
+
 ## documents
 
 这里存放的是 Armbian 使用文档。

build-armbian/README.md

@@ -8,6 +8,8 @@ The files needed to compile Armbian are stored in the relevant directory.
 
 The files stored here are related files that need to be used when packaging Armbian. The `common-files` directory is a general file, and the `platform-files` directory is a differentiated file for each platform.
 
+The required firmware will be automatically downloaded from the [ophub/firmware](https://github.com/ophub/firmware) repository to the `common-files/usr/lib/firmware` directory.
+
 ## documents
 
 Here are the Armbian documentation.