Add more detailed contribution guidelines

CONTRIBUTING.md

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+# Contributing
+
+If you discover issues, have ideas for improvements or new features,
+please report them to the [issue tracker][1] of the repository or
+submit a pull request. Please, try to follow these guidelines when you
+do so.
+
+## Issue reporting
+
+* Check that the issue has not already been reported.
+* Check that the issue has not already been fixed in the latest code
+  (a.k.a. `master`).
+* Be clear, concise and precise in your description of the problem.
+* Open an issue with a descriptive title and a summary in grammatically correct,
+  complete sentences.
+* Include the versions of Chewy, Elasticsearch, Ruby, Rails, etc.
+* Include any relevant code to the issue summary.
+
+## Pull requests
+
+* Read [how to properly contribute to open source projects on GitHub][2].
+* Fork the project.
+* Use a topic/feature branch to easily amend a pull request later, if necessary.
+* Write [good commit messages][3].
+* Use the same coding conventions as the rest of the project.
+* Commit and push until you are happy with your contribution.
+* If your change has a corresponding open GitHub issue, prefix the commit message with `[Fix #github-issue-number]`.
+* Make sure to add tests for it. This is important so I don't break it
+  in a future version unintentionally.
+* Add an entry to the [Changelog](CHANGELOG.md).
+* Please try not to mess with the Rakefile, version, or history. If
+  you want to have your own version, or is otherwise necessary, that
+  is fine, but please isolate to its own commit so I can cherry-pick
+  around it.
+* Make sure the test suite is passing and the code you wrote doesn't produce
+  RuboCop offenses.
+* [Squash related commits together][5].
+* Open a [pull request][4] that relates to *only* one subject with a clear title
+  and description in grammatically correct, complete sentences.
+
+## Changelog entry format
+
+Here are a few examples:
+
+```
+* [#753](https://github.com/toptal/chewy/pull/753): Add support for direct_import parameter to skip objects reloading. ([@TikiTDO][], [@dalthon][])
+* [#739](https://github.com/toptal/chewy/pull/739): Remove explicit `main` branch dependencies on `rspec-*` gems after `rspec-mocks` 3.10.2 is released. ([@rabotyaga][])
+```
+
+* Mark it up in [Markdown syntax][6].
+* The entry line should start with `* ` (an asterisk and a space).
+* If the change has a related GitHub issue (e.g. a bug fix for a reported issue), put a link to the issue as `[#123](https://github.com/toptal/chewy/issues/123): `.
+* Describe the brief of the change. The sentence should end with a punctuation.
+* If this is a breaking change, mark it with `**(Breaking)**`.
+* At the end of the entry, add an implicit link to your GitHub user page as `([@username][])`.
+* If this is your first contribution to the project, add a link definition for the implicit link to the bottom of the changelog as `[@username]: https://github.com/username`.
+
+[1]: https://github.com/toptal/chewy/issues
+[2]: https://www.gun.io/blog/how-to-github-fork-branch-and-pull-request
+[3]: https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+[4]: https://help.github.com/articles/about-pull-requests
+[5]: http://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html
+[6]: https://daringfireball.net/projects/markdown/syntax