Update README

When rpms_path is specified, one must also include modify_dir_path
due to this task:

- name: Copy RPMs to context dir
  copy:
    src: "{{ item }}"
    dest: "{{ modify_dir_path }}"
  with_list: "{{ rpms_list }}"
  when: rpms_path is defined

Change-Id: Ib4856722a79d01329295727ac6381dd7ffb829f9

README.rst

@@ -199,6 +199,7 @@ In this playbook the tasks\_from is set as a variable instead of an
           modified_append_tag: updated
           container_build_tool: buildah # or docker
           yum_cache: /tmp/containers-updater/yum_cache
+          modify_dir_path: /path/to/example_modify_dir
           rpms_path: /home/stack/rpms
 
 .. code-block::
@@ -213,6 +214,7 @@ In this playbook the tasks\_from is set as a variable instead of an
           source_image: docker.io/tripleomaster/centos-binary-nova-api:latest
           modified_append_tag: updated
           container_build_tool: docker # or buildah
+          modify_dir_path: /path/to/example_modify_dir
           rpms_path: /home/stack/rpms/
 
 Note, if you have a locally installed gating repo, you can add