Add CONTRIBUTING guide and code of conduct

[haesleinhuepf]

We should list criteria describing what things will be accepted as PR, and what not. Starting point for discussion:

We accept:

• PRs introducing new simple or complex tasks commonly done by bio-image analysts
• Tasks can be from any common workflow step in image processing, segmentation, feature extraction, tabular data wranging, statistics, dimensionality reduction, clustering
• Tasks involving multiple example image files or folder (see example_data folder)
• Tricky multi-step workflows are explicitly welcome, also if workflow designers suspect that no LLM can solve them at this point.
• Imaging modalities such as brightfield microscopy, fluorescence microscopy, electron microscopy, histology/histopathology are welcome
• Functional correctness should be evaluated on simple (small) example data.

We do not accept:

• new imaging modalities that are not commonly referred to as bio-image analysis (example: medical imaging, CT, MRI, astronomy, material science EM)
• new requirements that do not run on common computers or require specific hardware (e.g. CUDA)
• new files in the example_data folder that are larger than a small number of megabytes. Tests should use minimal amount of data
• new test-cases that need long run-time (approx > 0.3 seconds)

[haesleinhuepf]

We can have a discussion in the PR linked above.