docs: Added base64 instructions

Author: clivewalkden

README.md

@@ -21,7 +21,19 @@ To use this action, you need to provide the necessary configuration details for
         certificate_name: ${{ secrets.VPN_CERTIFICATE_NAME }}
 ```
 
-Make sure to store your sensitive information (such as the config, certificate, and certificate_name) as secrets in your GitHub repository to keep them secure.
+Make sure to store your sensitive information (such as the config, certificate, and certificate_name) as secrets in your GitHub repository to keep them secure. 
 
-For more information on how to use this action, please refer to the [GitHub Action documentation](https://docs.github.com/actions).
+The certificate will need to be converted to base 64 before you can save the value as a GitHub secret.
+
+```shell
+# Mac/Linux
+base64 <file_name> > output.txt
+
+# Windows
+certutil -encode <file_name> output.txt
+```
+Replace `<file_name>` with the name of the file you want to encode. The encoded output will be saved in `output.txt`.
+
+Remember to replace `<file_name>` with the actual name of the file you want to encode.
 
+For more information on how to use this action, please refer to the [GitHub Action documentation](https://docs.github.com/actions).