add constraints file to requirements-txt-fixer files
diff --git a/.pre-commit-hooks.yaml b/.pre-commit-hooks.yaml
index f8523d4..c0d811c 100644
--- a/.pre-commit-hooks.yaml
+++ b/.pre-commit-hooks.yaml
@@ -185,7 +185,7 @@
     description: sorts entries in requirements.txt.
     entry: requirements-txt-fixer
     language: python
-    files: requirements.*\.txt$
+    files: (requirements|constraints).*\.txt$
 -   id: sort-simple-yaml
     name: sort simple yaml files
     description: sorts simple yaml files which consist only of top-level keys, preserving comments and blocks.
diff --git a/README.md b/README.md
index 3b3b330..9751c00 100644
--- a/README.md
+++ b/README.md
@@ -179,7 +179,7 @@
   - `--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`
+Sorts entries in requirements.txt and constraints.txt and removes incorrect entry for `pkg-resources==0.0.0`
 
 #### `sort-simple-yaml`
 Sorts simple YAML files which consist only of top-level