Add README

This README should help to inspire people to run this themselves. It
will introduce people on how to set this up and provide a nice
integration with their GitLab instance.

README.md

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+Renovate Bot
+===
+
+This repository is a GitLab repository to run Renovate Bot in a self-hosted GitLab instance via GitLab CI. It provides basic features for dependency updates.
+
+Usage
+---
+
+On the SI-GitLab instance authorize the user Botaniker to have access to your repository. The current schedule for this to run is once a week. You'll get a setup MR from Botaniker within a week. Once it's merged, it'll automatically check your dependencies and inform you about updates.
+
+If you want to run this on your own GitLab instance:
+
+1. Clone this repository and set it up on your own GitLab instance.
+2. Create a user for the bot and create and [API access token](https://git.shivering-isles.com/help/user/profile/personal_access_tokens.md).
+3. Go to Settings -> CI/CD -> Variables of your `renovate bot`-repository. Add a variable called [`RENOVATE_TOKEN`](https://github.com/renovatebot/renovate/blob/master/docs/development/self-hosting.md#gitlab-ceee) and fill it with the access token from step 2.
+4. Setup a [GitLab CI schedule](https://git.shivering-isles.com/help/user/project/pipelines/schedules.md) for the `renovate bot`-repository to run on a regular basis.
+5. Grant your bot user access to repositories you want to have checked.
+
+More information
+---
+
+- Check the official [renovate bot page](https://renovatebot.com/).
+- See the docs about how to [tweak the renovate bot config files](https://docs.renovatebot.com/configuration-options/)
+- Feel free to [report issues here](https://git.shivering-isles.com/shivering-isles/renovate-bot/issues)