docs(readme): add dockerfile flag documentation (#1566)

README.md

@@ -59,6 +59,7 @@ _If you are interested in contributing to kaniko, see [DEVELOPMENT.md](DEVELOPME
     - [--context-sub-path](#--context-sub-path)
     - [--customPlatform](#--customPlatform)
     - [--digest-file](#--digest-file)
+    - [--dockerfile](#--dockerfile)
     - [--force](#--force)
     - [--git](#--git)
     - [--image-name-with-digest-file](#--image-name-with-digest-file)
@@ -597,6 +598,10 @@ will write the digest to that file, which is picked up by
 Kubernetes automatically as the `{{.state.terminated.message}}`
 of the container.
 
+#### --dockerfile
+
+Path to the dockerfile to be built. (default "Dockerfile")
+
 #### --force
 
 Force building outside of a container