Add CONTRIBUTING.md

[sunjay]

• Reporting bugs (put this section first)
  • Provide the code that doesn't work
  • Describe what the problem is in enough detail for us to understand what you're talking about and why it's a problem
  • Provide the version of Rust (run rustc -V) and the version of the turtle crate (found in Cargo**.lock**)
  • Optional: Search within the existing issues to see if your problem has already been reported (make sure to check closed issues too)
  • Optional: try to reduce the amount of code in your example of the problem to only the minimum amount needed to produce the problem (this helps us narrow things down)
• Fixing bugs (opening a PR, include what issue you're fixing and maybe a small description of what you did to fix it)
  • Feel free to ask if you want to fix something but don't know how
  • We may be able to provide you with some rough instructions or at least tell you where to look
• Overview of basic architecture (two threads, animation, drawing commands, etc.) (See Provide an overview of how turtle works #90)
• Build commands
• Test commands
  • You must use cargo test --features "test"
  • Animations and the window opening are disabled during tests so setting the speed will have no impact and you should not test things that depend on that from within unit tests
• Link to CONTRIBUTING.md from README.md
• Consider using rustfmt
• explain issue labels
  • help wanted labels are the best labels to attempt (though any issue can be attempted if you are up for it)
  • issues labelled "good first issue" often have "help wanted" as well - these are good issues for people to start with
  • issues labelled "help wanted" without "good first issue" are often much more involved, still doable, but require much more work and knowledge usually (feel free to ask anyway about why an issue might be a bit harder)
• add some stuff about being gracious and accepting feedback during code reviews: https://truss.works/blog/2018/1/5/of-tracked-changes-and-diffs-moving-from-editorial-to-engineering

Example: https://github.com/atom/atom/blob/master/CONTRIBUTING.md

Ask people not to ask questions in issues. Send them to stack overflow or something.