https://github.com/scikit-learn/scikit-learn/blob/doc/developers/contributing.rst currently mixes a lot of discussion of how to contribute and how to develop scikit-learn compatible stuff. I think we should separate out "Contributing" from "Developing estimators" (this should become a new file in doc/developers). I'm not sure which comes first, and there is some content that would be hard to place.

This is the current TOC with a suggestion for where each section belongs (up for discussion):

• Ways to contribute - Contributing
• Submitting a bug report or a feature request - Contributing
  • How to make a good bug report
• Contributing code - Contributing
  • How to contribute
  • Pull request checklist
    • Continuous Integration (CI)
    • Stalled pull requests
  • Issues for New Contributors
• Documentation - Contributing
  • Building the documentation
  • Guidelines for writing documentation
  • Generated documentation on CircleCI
• Testing and improving test coverage - Developing
  • Writing matplotlib related tests
  • Workflow to improve test coverage
  • Issue Tracker Tags - Contributing - I don't think this belongs here in any case
• Coding guidelines - Contributing for top matter
  • Input validation - Developing?
  • Random Numbers - Developing
  • Deprecation - Contributing
  • Change the default value of a parameter - Contributing
  • Python versions supported - Contributing
• Code Review Guidelines - Contributing
• APIs of scikit-learn objects - Developing
  • Different objects
  • Estimators
    • Instantiation
    • Fitting
    • Estimated Attributes
    • Optional Arguments
    • Pairwise Attributes
• Rolling your own estimator - Developing
  • get_params and set_params
  • Parameters and init
  • Cloning
  • Pipeline compatibility
  • Estimator types
  • Specific models
  • Estimator Tags
    Reading the existing code base - Contributing