Add CONTRIBUTING guide

# How to contribute

## Add a new services

Open the file [`api/v2/services.json`](./api/v2/services.json), add a comma after the last entry, and then add a line like this:

```json

{"name": "Cooler Fachschaftsservice", "host": "service.fachschaft.cl.uni-heidelberg.de", "category": "fachschaft"}

```

> [!warning]

>

> JSON doesn't allow a trailing `,` after the last entry, so:

>

> 1. You must add one to the preceding entry before adding your new entry

> 2. You must make sure _not_ to add one after your new entry

If the service is behind HTTP Basic Auth, add a line like this:

```json

{"name": "Cooler Fachschaftsservice hinter Auth", "host": "service.fachschaft.cl.uni-heidelberg.de", "category": "fachschaft", "expectUnauthorized": true}

```

## Change something else about the code

The [README](README.md) has quite a detailed architecture description, read it, and then do whatever you want to.

## Deploy code

Anything on `master` is automatically deployed to status.fsco.li using a CI/CD pipeline.