Add Renovate config

[ysangkok]

I noticed that @domenkozar was interested in automated updates.
I am looking for a real-world repo to showcase stack.yaml support for.

So I thought I would submit this here since there seems to be interest.

This PR should work as submitted after setting up the Github app.

For local testing, the following configuration options need to be added:

, "onboarding": false
, "token": "<secret github token>"
, "repositories": [{ repository: 'yourGithubUser/websockets' }]
, "platform": "github"

Renovate can then be executed locally using:

docker run --mount type=bind,src=$PWD,dst=/mount-point,ro \
  -e RENOVATE_REQUIRE_CONFIG=ignored \
  -e RENOVATE_CONFIG_FILE=/mount-point/renovate.json5 \
  -e LOG_LEVEL=debug \
  renovate/renovate

The advantage of this setup, which loads the configuration from your local
filesystem, is that you can iterate without making repo changes.