Configure Digitransit CI actions

Author: VillePihlava

.github/CODEOWNERS

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+<h1 align="center">
+  <img src="/doc/user/images/otp-logo.svg" width="120" /><br>
+  OpenTripPlanner (OTP) - Digitransit Fork
+</h1>
+<p align="center">
+  <b>
+    HSL OTP Deployments ・ 
+    <a href="https://dev.reittiopas.fi/etusivu">DEV</a> ・
+    <a href="https://reittiopas.hsl.fi/etusivu">PROD</a>
+  </b>
+</p>
+<p align="center">
+  <b>
+    HSL OTP Debug ・ 
+    <a href="https://dev-hsl-debug.digitransit.fi/">DEV</a> ・
+    <a href="https://hsl-debug.digitransit.fi/">PROD</a>
+  </b>
+</p>
+<p align="center">
+  <b>
+    OTP GraphQL Explorer ・ 
+    <a href="https://dev-api.digitransit.fi/graphiql/hsl">DEV</a> ・
+    <a href="https://api.digitransit.fi/graphiql/hsl">PROD</a>
+  </b>
+</p>
+
+# Overview of CI and Branch Configuration
+
+This is the Digitransit fork of [OpenTripPlanner](https://github.com/opentripplanner/OpenTripPlanner).
+The [`digitransit_ext_config`](https://github.com/HSLdevcom/OpenTripPlanner/tree/digitransit_ext_config) branch contains the Digitransit CI configuration and scripts.
+
+> **The contents of the `v2` branch are reset often and nothing is configured to be retained in the branch permanently!
+It should not be used as a base for changes.
+Always use the upstream [`dev-2.x`](https://github.com/opentripplanner/OpenTripPlanner) branch as a base and make contributions there.**
+
+## Relevant Branches
+
+This repository contains the current dev version in-use in the `v2` branch.
+The current prod version comes from the [latest release](https://github.com/HSLdevcom/OpenTripPlanner/releases/latest).
+There is also a `custom-release` branch that is used for building versions of OTP that differ from the `v2` branch.
+
+## CI Configuration
+
+Checkout [`.github/workflows`](/.github/workflows/). The configured CI pipelines build docker images for dev and prod. The built images can be found here: https://hub.docker.com/r/hsldevcom/opentripplanner.
+
+## [`merge_upstream.sh`](https://github.com/HSLdevcom/OpenTripPlanner/blob/digitransit_ext_config/merge_upstream.sh)
+
+This script can be used to merge and push changes from e.g. `upstream/dev-2.x` to the `v2` or `custom-release` branches.
+You need to have `python3` and `jq` installed.
+
+## [`digitransit/RELEASE_CHANGELOG.md`](/digitransit/RELEASE_CHANGELOG.md)
+
+The generated changelog diff can be found in this file, if it exists.