docs: improve cross-platform clipboard support for key copying

- Add instructions to install `xclip` on Ubuntu before copying the private key
- Replace `clip` command with platform-specific commands (`pbcopy` for macOS and `xclip` for Ubuntu) for copying the private key

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

README.md

@@ -122,14 +122,29 @@ Copy Private Key content and paste in Github Secrets.
 
 ### Copy rsa Private key
 
+Before copying the private key, install `clip` command as shown below:
+
+```bash
+# Ubuntu
+sudo apt-get install xclip
+```
+
+copy the private key:
+
 ```bash
-clip < ~/.ssh/id_rsa
+# macOS
+pbcopy < ~/.ssh/id_rsa
+# Ubuntu
+xclip < ~/.ssh/id_rsa
 ```
 
 ### Copy ed25519 Private key
 
 ```bash
-clip < ~/.ssh/id_ed25519
+# macOS
+pbcopy < ~/.ssh/id_ed25519
+# Ubuntu
+xclip < ~/.ssh/id_ed25519
 ```
 
 See the detail information about [SSH login without password](http://www.linuxproblem.org/art_9.html).