Add a simple CONTRIBUTING.md guide

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+# Contributing
+
+First and foremost: **thank you very much** for your interest in this
+project. Feel free to skip all this and open your issue / pull request
+if reading contribution guidelines is too much for you at this point.
+We value your contribution a lot more than we value your ability to
+follow rules (and thankfully we can afford to take this approach given
+this project's demand).
+
+Any kind of contribution is welcome. We can always use better docs and
+tests (and code, of course). If you think you can improve this project
+in any dimension _let's talk_ :-)
+
+## Guidelines
+
+Be kind and respectful in all your interactions with people inside
+(outside too!) this community; There is no excuse for not showing
+basic decency. Sarcasm and generally unconstructive remarks are **not
+welcome**.
+
+### Issues
+
+When opening and interacting with issues please:
+
+- Be as clear as possible
+- Provide examples if you can
+
+### Pull Requests
+
+We expect that pull requests:
+
+- Have [good commit messages][commits]
+- Contain tests for new features
+- Target and can be cleanly merged with the `master` branch
+- Pass the tests (TravisCI will run them automatically but `go test` is
+ your friend)
+
+[commits]: https://www.git-scm.com/book/en/v2/Distributed-Git-Contributing-to-a-Project#_commit_guidelines
+
+### Project Management
+
+Don't bother with labels, milestones, assignments, etc. We don't make
+use of those.

caio.co/de/go-tdigest

Created CONTRIBUTING.md