Add CONTRIBUTING advice

CONTRIBUTING.md

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+# Contributing
+
+Welcome to the project!
+
+This file is a short guide on how to contribute to it.
+The gist is: just go ahead and hack at it.
+
+## Feature Requests, Enhancements, Bugs, Issues
+
+Please feel free to submit issues about problems and bugs you encountered.
+
+Features and enhancements you would like to see added can also go there, and we can
+then see towards implementing them for you.
+Chances are that what you seek is already possible with the (many) included packages,
+and only an example in the cookbook showing how to do it (but no change to the `*.cls`
+file) is needed.
+
+## Developing Environment
+
+The environment for compilation is taken care of by the Docker image as specified in
+[the CI config file](.gitlab-ci.yml) (search for the `image` name on
+[Dockerhub](https://hub.docker.com/)).
+Refer to its `DOCKERFILE` for what it does exactly.
+
+It would be greatly beneficial to use that Docker image for yourself.
+It is guaranteed and proven to work (a succeeding pipeline implies a succeeding Docker
+image).
+Otherwise, it is not terribly hard to set up a local environment imitating that Docker
+image.
+For installation advice, refer to the cookbook PDF Preface chapter.
+
+## Style
+
+Being a LaTeX project, there are no generally agreed-upon guides, styles, best
+practices and the like.
+Everything is basically free-form.
+A look at the source code will reveal the few (stylistic) choices chosen for this project:
+
+- A line limit of about 90.
+- Proper indentation (this is very important!) for readability and maintainability.
+ This is also true for math environments, where indenting `\frac{}{}` and other
+ multi-argument commands will do wonders for legibility.
+ Proper indentation involves aligning opening and closing braces and all that jazz.
+ Unfortunately, there is no formatter to automatically do this for LaTeX.
+- Proper commenting and even links to stackexchange solutions will probably be helpful
+ and avoid duplicate lookups of already-solved issues.