Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Contributions of all sizes are welcomed, whether it's a typo fix in documentation, a bug fix proposal, feature request or completely a new idea for the project.
+
+## Report Bugs
+
+Report bugs in the [issue tracker](https://github.com/Miksus/red-mail/issues). Please describe how the bug occurred, what 
+the bug was and, if relevant, what email provider you used and what versions (Python and OS) you used.
+
+## Feature Requests and Ideas for Docs Improvements
+
+You may submit your ideas and documenation improvements using the [issue tracker](https://github.com/Miksus/red-mail/issues).
+Please use the appropriate issue template.
+
+## Pull Requests
+
+Also code contributions are welcome. For bug fixes and feature requests, it is recommended to first create an issue describing the 
+problem the idea. For simple typo fixes in documentation or in docstrings creating issues are not necessary.
+
+To create a pull request:
+
+- Create an issue (unless a trivial and simple change)
+- Fork the repository
+- Do your changes
+- If you did code changes, run the tests using tox:
+  ```python
+  pip install tox
+  python -m tox
+  ```
+- If you did documantation changes, build the documentation using tox:
+  ```python
+  pip install tox
+  python -m tox -e docs
+  ```
+- Make a pull request
+
+If you made code changes:
+
+- Test your code completely. The target is at 100 % test coverage.
+- Write some documentations (if new feature). The target is to document 100 % of the code.
+- Write code that is understandable, clear and follow the chosen conventions. 
+  Read the code base the gain an idea of the existing style and try to follow it.
+    - Follow PEP-8 as much as possible
+    - Use Numpydocs for docstrings
+    - Avoid hacks and unreliable solutions
+
+If you improved documentation:
+
+- Make sure the docs gets built without an error
+- Make sure the part you changed looks OK also when rendered as HTML
+
+You can find the built documentation at: ``redmail/docs/_build/html``