docs: update README with private key instructions in multiple languages

- Add instructions for copying the private key to GitHub Secrets in English, Chinese (Simplified), and Chinese (Traditional) versions of the README.

ref: https://github.com/appleboy/ssh-action/issues/6#issuecomment-922368411
Signed-off-by: appleboy <appleboy.tw@gmail.com>

README.md

@@ -138,6 +138,8 @@ pbcopy < ~/.ssh/id_rsa
 xclip < ~/.ssh/id_rsa
 ```
 
+Starting from and including the comment section `-----BEGIN OPENSSH PRIVATE KEY-----` and ending at and including the comment section `-----END OPENSSH PRIVATE KEY-----`, copy the private key and paste it into GitHub Secrets.
+
 ### Copy ed25519 Private key
 
 ```bash

README.zh-cn.md

@@ -138,6 +138,8 @@ pbcopy < ~/.ssh/id_rsa
 xclip < ~/.ssh/id_rsa
 ```
 
+从包含注释部分 `-----BEGIN OPENSSH PRIVATE KEY-----` 开始，到包含注释部分 `-----END OPENSSH PRIVATE KEY-----` 结束，复制私钥并将其粘贴到 GitHub Secrets 中。
+
 ### 复制 ed25519 私钥内容
 
 ```bash

README.zh-tw.md

@@ -136,6 +136,8 @@ pbcopy < ~/.ssh/id_rsa
 xclip < ~/.ssh/id_rsa
 ```
 
+從包含註釋部分 `-----BEGIN OPENSSH PRIVATE KEY-----` 開始，到包含註釋部分 `-----END OPENSSH PRIVATE KEY-----` 結束，複製私鑰並將其粘貼到 GitHub Secrets 中。
+
 ### 複製 ed25519 私鑰內容
 
 ```bash