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

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

website/docs/usage/args.md

@@ -36,7 +36,7 @@ git-cliff [FLAGS] [OPTIONS] [--] [RANGE]
     --tag-pattern <PATTERN>      Sets the regex for matching git tags [env: GIT_CLIFF_TAG_PATTERN=]
     --with-commit <MSG>...       Sets custom commit messages to include in the changelog [env: GIT_CLIFF_WITH_COMMIT=]
     --skip-commit <SHA1>...      Sets commits that will be skipped in the changelog [env: GIT_CLIFF_SKIP_COMMIT=]
--p, --prepend <PATH>             Prepends entries to the given changelog file [env: GIT_CLIFF_PREPEND=]
+-p, --prepend <PATH>             Prepends entries to the given changelog file, incompatible with -o [env: GIT_CLIFF_PREPEND=]
 -o, --output [<PATH>]            Writes output to the given file [env: GIT_CLIFF_OUTPUT=]
 -t, --tag <TAG>                  Sets the tag for the latest version [env: GIT_CLIFF_TAG=]
 -b, --body <TEMPLATE>            Sets the template for the changelog body [env: GIT_CLIFF_TEMPLATE=]