Output subflow information to README.md

[kazuhitoyokoi]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

I found that the generated README.md file doesn't contain the subflow node information in exporting subflow functionality. Therefore, I fixed it in this pull request.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality