Document auto_tag behaviour more explicite (#37)

close #31

Co-authored-by: Lauris BH <lafriks@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/37
Reviewed-by: Lauris BH <lafriks@noreply.codeberg.org>

docs.md

@@ -40,6 +40,12 @@ It will automatically generate buildkit configuration to use custom CA certifica
 | `tag`/`tags`              | @".tags"          | sets repository tags to use for the image
 | `platforms`               | *none*            | sets target platform for build
 
+## auto_tag
+
+If set to ture, it will use the `default_tag` ("latest") on tag event or default branch.
+If it's a tag event it will also asume sem versioning and add taggs acordingly (`x`, `x.x` and `x.x.x`).
+If it's not a tag event, and no default branch, automated tags are skipped.
+
 ## Examples
 
 ```yml