[docs] Contributing file (#575)

- [ci skip]

CONTRIBUTING.md

@@ -0,0 +1,60 @@
+# How to contribute
+
+Bug reports and pull requests from users is what keeps this project working.
+
+## Basics
+
+1. Create an issue and describe your idea
+2. [Fork it](https://github.com/skywinder/github-changelog-generator/fork)
+3. Create your feature branch (`git checkout -b my-new-feature`)
+4. Commit your changes (`git commit -am 'Add some feature'`)
+5. Publish the branch (`git push origin my-new-feature`)
+6. Create a new Pull Request
+
+## Checking your work
+
+You can test your workflow with changelog generator with
+[the skywinder/changelog_test repo].
+
+You can run the test suite.
+
+You can run [RuboCop] to check code style.
+
+The default Rake task, runnable using `rake`, calls `rubocop`, then `spec`.
+
+[the skywinder/changelog_test repo]: https://github.com/skywinder/changelog_test/
+[RuboCop]: http://rubocop.readthedocs.io/en/latest/
+
+## Write documentation
+
+This project has documentation in a few places:
+
+### Introduction and usage
+
+A friendly `README.md` written for many audiences.
+
+### Examples and advanced usage
+
+The [wiki].
+
+###  API documentation 
+
+API documentation is written as [YARD] docblocks in the Ruby code.
+
+This is rendered as Web pages on [Rubydoc.info][github-changelog-generator on Rubydoc.info].
+
+The completeness of the API documentation is measured on [our page on the Inch CI Web site][github-changelog-generator on Inch CI]
+
+### man page
+
+`man/git-generate-changelog.md`
+
+The man page is for the `git generate-changelog` Git sub-command, which is a wrapper for `github_changelog_generator`. That file is a Markdown file.
+
+Use the [ronn] gem to generate `.1` and `.html` artifacts like this: `cd man; ronn git-generate-changelog.md`
+
+[wiki]: https://github.com/skywinder/github-changelog-generator/wiki
+[YARD]: https://yardoc.org/
+[github-changelog-generator on Rubydoc.info]: http://www.rubydoc.info/gems/github_changelog_generator
+[ronn]: https://github.com/rtomayko/ronn
+[github-changelog-generator on Inch CI]: https://inch-ci.org/github/skywinder/github-changelog-generator

README.md

@@ -270,15 +270,9 @@ can't get the latest version of Ruby installed.
 
 ## Contributing
 
-1. Create an issue and describe your idea
-2. [Fork it](https://github.com/skywinder/github-changelog-generator/fork)
-3. Create your feature branch (`git checkout -b my-new-feature`)
-4. Commit your changes (`git commit -am 'Add some feature'`)
-5. Publish the branch (`git push origin my-new-feature`)
-6. Create a new Pull Request
-7. Profit! :white_check_mark:
+We have collected notes on how to contribute to this project in [CONTRIBUTING.md].
 
-You can test your workflow with changelog generator with [the skywinder/changelog_test repo](https://github.com/skywinder/changelog_test/).
+[CONTRIBUTING.md]: CONTRIBUTING.md
 
 ## License