Add your script as a multi-line YAML string.

steps:
  - name: List Pull Request files
    plugin: Script
    script: |
      cd $SRC_DIR
      files=$(ls)
      echo "Listing all files included in this Pull Request..."
      echo files $files

You can also add your script in the traditional YAML syntax. Each new line in your script is a new item in the sequence.

steps:
  - name: List Pull Request files
    plugin: Script
    script:
      - cd $SRC_DIR
      - files=$(ls)
      - echo "Listing all files included in this Pull Request..."
      - echo files $files