Add docs and a CHANGELOG entry for --only option

Author: MatheusRich

CHANGELOG.md

@@ -1,5 +1,7 @@
 ## [Unreleased]
 
+- Add `--only` option to only include specific files or directories in the diff.
+
 ## [0.5.0] - 2025-03-10
 
 - Don't abort process on bundle check failure.

README.md

@@ -96,6 +96,14 @@ Skip specific files or directories during the diff.
 rails-diff generated scaffold Post --skip app/views app/helpers
 ```
 
+#### --only <files/directories>
+
+Only include specific files or directories in the diff.
+
+```bash
+rails-diff generated scaffold Post --only app/models app/controllers
+```
+
 #### --clear-cache/--no-cache
 
 Clear the cache directory to force cloning Rails and regenerating the Rails template app.