added additional flag for base64 encoding under pipectl encrypt

Signed-off-by: antedotee <soniyadav2051982@gmail.com>

Author: antedotee

docs/content/en/docs-dev/user-guide/command-line-tool.md

@@ -355,6 +355,9 @@ You can encrypt it the same way you do [from the web](../managing-application/se
       --input-file={PATH_TO_SECRET_FILE}
   ```
 
+Optional encoding:
+- `--use-base64-encoding` to base64-encode the plaintext before encrypting.
+
 Note: The docs for pipectl available command is maybe outdated, we suggest users use the `help` command for the updated usage while using pipectl.
 
 ### Generating an application config (app.pipecd.yaml)