Sync-Bot

Set up a GitHub Action to sync my fork's main branch with their upstream original.

• use https://github.com/wei/git-sync to do the synchronization
• create an SSH2 private/public keypair
  • put the private key as SSH_PRIVATE_KEY secret in the sync-bot repo
  • put the public key as username > settings > ssh authentication key (not as the Repo > Deploy Key, like the action README suggests)