Add CONTRIBUTING.md

CONTRIBUTING.md

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+## How to contribute to this project
+
+### Casual contributors
+
+#### Create issues
+1. mention the version number of the package
+1. write at least one sentence describing the issue
+1. share the output of `versioninfo()`
+1. indicate if it is a bug report or feature request
+
+### Regular contributors
+1. become member of the Telegram channel
+1. ask for write access
+
+#### Creating pull requests
+1. create a new branch
+1. push your changes to the branch
+1. create a draft pull request
+1. when you think it is ready, mark it as "ready for review" and invite a reviewer
+1. address the remarks of the reviewer
+1. check that all tests passed
+1. merge the pull request, choosing the `squash` option
+
+It is OK to merge your own pull request without review if you are very sure it will not cause any problems. If in doubt, ask on Telegram.
+
+It is OK to push directly to the `main` branch for small changes for example fixing a typo in the documentation.
+
+#### Commit messages
+1. shall start with capital letter
+1. imperative form
+
+**Examples**
+- Fix division-by-zero error
+- Improve docstrings
+- Implement feature #23
+- Add function cool_feature()
+
+#### Coding style
+1. variable and function names lower case, use underscore if needed
+1. Types and structs CamelCase
+1. indent with four spaces
+1. max 120 char per line
+1. do NOT use an automated code formatter
+
+#### File formats
+1. input files: preferably `.yaml`format
+1. CAD geometries: `.obj` files
+1. multi-dimensional arrays: `.mat` files
+1. CSV files can be used for polars for example, but if the polars are part of wing description better have one `.yaml` file that describes all aspects of the wing
+
+**Open questions** 
+- what are `.dat` files?
+- shall we use `.bin` files?
+- shall we use `.JLD2` files?