Add a contributing document

CONTRIBUTING.md

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+## How to contribute to IDArling
+
+*This contributing file is modified from [Ruby on Rails](https://github.com/rails/rails/blob/master/CONTRIBUTING.md)*
+
+### Quick guidelines for commits
+
+* Write cross-platform code. IDArling is working for both macOS, Linux and Windows. Make sure you are not using things like `os.system("ls")` directly.
+* Split your large commit into several parts. As we have [discussed](https://github.com/IDArlingTeam/IDArling/issues/46#issuecomment-418980108), using several small commits is easier to see the history through `git log`. If you do not know how to selectively `git add` your file, go stackoverflow or see [this](https://mirrors.edge.kernel.org/pub/software/scm/git/docs/git-add.html#_interactive_mode).
+* Test before your PR. Make sure your commit does not break anything, and can pass code style check. We use `black` and `flake8`. See [travis config file](https://github.com/IDArlingTeam/IDArling/blob/master/.travis.yml) for more.
+
+### Did you find a bug?
+
+* **Ensure the bug was not already reported** by searching on GitHub under [Issues](https://github.com/IDArlingTeam/IDArling/issues).
+
+* If you're unable to find an open issue addressing the problem, [open a new one](https://github.com/IDArlingTeam/IDArling/issues/new). Be sure to include a **title and clear description**, as much relevant information as possible, and a **code sample** or an **executable test case** demonstrating the expected behavior that is not occurring.
+
+### Did you write a patch that fixes a bug?
+
+* Open a new GitHub pull request with the patch.
+
+* Ensure the PR description clearly describes the problem and solution. Include the relevant issue number if applicable.
+
+* Before submitting, please read the **Quick guidelines for commits** above for coding guidelines.
+
+### Do you intend to add a new feature or change an existing one?
+
+* Suggest your change by opening a new issue, and start writing code in your own fork.
+
+* New features or improvements will be discussed in the issue, and be sure you get enough favors before you open a new Pull Request.
+
+#### **Do you want to contribute to the IDArling documentation?**
+
+* Yes, we do need you to write it. You can be the first one to write it!
+
+Thanks! :heart: :heart: :heart:
+
+IDArling Team