Add note about sort-simple-yaml's opt-in behaviour
diff --git a/README.md b/README.md
index 1950fa1..584b53d 100644
--- a/README.md
+++ b/README.md
@@ -94,7 +94,18 @@
- `--no-sort-keys` - when autofixing, retain the original key ordering (instead of sorting the keys)
- `--top-keys comma,separated,keys` - Keys to keep at the top of mappings.
- `requirements-txt-fixer` - Sorts entries in requirements.txt and removes incorrect entry for `pkg-resources==0.0.0`
-- `sort-simple-yaml` - Sorts simple YAML files which consist only of top-level keys, preserving comments and blocks.
+- `sort-simple-yaml` - Sorts simple YAML files which consist only of top-level
+ keys, preserving comments and blocks.
+
+ Note that `sort-simple-yaml` by default matches no `files` as it enforces a
+ very specific format. You must opt in to this by setting `files`, for
+ example:
+
+ ```yaml
+ - id: sort-simple-yaml
+ files: ^config/simple/
+ ```
+
- `trailing-whitespace` - Trims trailing whitespace.
- To preserve Markdown [hard linebreaks](https://github.github.com/gfm/#hard-line-break)
use `args: [--markdown-linebreak-ext=md]` (or other extensions used
