docs(website): add note about incompatibility with output to prepend arg

Signed-off-by: Anton Engelhardt <antoncengelhardt@icloud.com>

website/docs/usage/examples.md

@@ -72,6 +72,7 @@ Prepend new changes to an existing changelog file:
 ```bash
 # 1- changelog header is removed from CHANGELOG.md
 # 2- new entries are prepended to CHANGELOG.md without footer part
+# the --prepend option is incompatible with -o (output)
 git cliff --unreleased --tag 1.0.0 --prepend CHANGELOG.md
 ```