doc: update README.md for macOS users (#1988)

Signed-off-by: leo <longshuang@msn.cn>

Author: love-linger

README.md

@@ -113,6 +113,9 @@ For **macOS** users:
   ```shell
   sudo xattr -cr /Applications/SourceGit.app
   ```
+> [!NOTE]
+> macOS packages in the `Release` page of this project are all unsigned. If you are worried about potential security issues with the above command, you can download the signed package from the distribution repository provided by [@ybeapps](https://github.com/ybeapps) (there is no need to execute the above command while installing `SourceGit`).
+
 * Make sure [git-credential-manager](https://github.com/git-ecosystem/git-credential-manager/releases) is installed on your mac.
 * You can run `echo $PATH > ~/Library/Application\ Support/SourceGit/PATH` to generate a custom PATH env file to introduce `PATH` env to SourceGit.