Add contributing guidelines

CONTRIBUTING.md

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+# Contributing
+
+First off, thanks for taking the time to contribute! :tada:
+
+## Bugs and questions
+
+Report bugs, ask questions, and request features using [GitHub Issues](https://github.com/johansatge/obsidian-automatic-table-of-contents/issues).
+
+When submitting a bug report, do not hesitate to be as exhaustive as possible, by adding:
+
+* A quick summary of the bug
+* The expected and actual behavior
+* The platform you are using (operating system, Obsidian version...)
+* A Markdown sample
+* Any other information that you think would be useful
+
+## Working on the project
+
+When contributing to this repository, please create an issue first, so the change you wish to make can be discussed with the other maintainers.
+
+When you submit code changes, your submissions are understood to be under the same license that covers the project. Feel free to contact the maintainers if that's a concern.