Automatically create and update running PR with changes from slack-api-ref

[duffn]

Every once in awhile there's a PR like this where somebody manually updates the codebase from slack-api-ref by running the update Rake task. #455

What about some automated process that runs maybe once per day (sometime after slack-api-ref), runs the Rake task, and creates or updates a PR in this repo with the changes generated.

• New changes but no PR -> open a PR with the changes
• New changes with already existing PR -> update the PR with the new changes
• No changes -> do nothing

This wouldn't actually merge anything to the main branch, but just maintain a running PR with changes from slack-api-ref. Maintainers of the repo could then merge the PR whenever they like. I'm thinking that this is something like the running PRs from https://github.com/googleapis/release-please

[dblock]

Love it. Add an automatic update of CHANGELOG (either simple that says "updated Slack API to [date]" or fancy "added x, y, z method" and those PRs will be mergeable. Also see #10 that was in that spirit.

[duffn]

Great. I'll take a go at this.