project: update TODO, README and add CONTRIBUTING

Let's make the lives of newcomers easier :)

CONTRIBUTING.md

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+# Contributing
+
+When contributing to the development of GNOME Calendar, please first discuss the change you wish to
+make via issue, email, or any other method with the maintainers before making a change.
+
+Please note we have a code of conduct, please follow it in all your interactions with the project.
+
+## Pull Request Process
+
+1. Ensure your code compiles. Run `make` before creating the pull request.
+2. If you're adding new API, it must be properly documented.
+3. You may merge the pull request in once you have the sign-off of the maintainers, or if you
+   do not have permission to do that, you may request the second reviewer to merge it for you.
+
+## Code of Conduct
+
+GNOME Calendar is a project developed based on GNOME Code of Conduct. You can read it below:
+
+### Summary
+
+GNOME creates software for a better world. We achieve this by behaving well towards
+each other.
+
+Therefore this document suggests what we consider ideal behaviour, so you know what
+to expect when getting involved in GNOME. This is who we are and what we want to be.
+There is no official enforcement of these principles, and this should not be interpreted
+like a legal document.
+
+### Advice
+
+ * **Be respectful and considerate**: Disagreement is no excuse for poor behaviour or personal
+     attacks. Remember that a community where people feel uncomfortable is not a productive one.
+
+ * **Be patient and generous**: If someone asks for help it is because they need it. Do politely
+     suggest specific documentation or more appropriate venues where appropriate, but avoid
+     aggressive or vague responses such as "RTFM".
+
+ * **Assume people mean well**: Remember that decisions are often a difficult choice between
+     competing priorities. If you disagree, please do so politely. If something seems outrageous,
+     check that you did not misinterpret it. Ask for clarification, but do not assume the worst.
+
+ * **Try to be concise**: Avoid repeating what has been said already. Making a conversation larger
+     makes it difficult to follow, and people often feel personally attacked if they receive multiple
+     messages telling them the same thing.
+
+
+In the interest of fostering an open and welcoming environment, we as
+contributors and maintainers pledge to making participation in our project and
+our community a harassment-free experience for everyone, regardless of age, body
+size, disability, ethnicity, gender identity and expression, level of experience,
+nationality, personal appearance, race, religion, or sexual identity and
+orientation.
+
+### Attribution
+
+This Code of Conduct is adapted from the [Contributor Covenant][homepage], version 1.4,
+available at [http://contributor-covenant.org/version/1/4][version]
+
+[homepage]: http://contributor-covenant.org
+[version]: http://contributor-covenant.org/version/1/4/