add new impact/distribution label

Adds the new label to the documentation. The
impact/distribution label is intended for changes
that affect the image-format or interaction
with the registry (distribution).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

project/REVIEWING.md

@@ -43,8 +43,9 @@ help in reviewing the PR, or because of the potential impact of the PR on their
  * `impact/api`
  * `impact/changelog`
  * `impact/cli`
- * `impact/dockerfile`
  * `impact/deprecation`
+ * `impact/distribution`
+ * `impact/dockerfile`
 
 # Workflow