Add contributing.md

Author: SnO2WMaN

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pages/contributing.md

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+# Contributing
+
+We welcome all kinds of contributions, questions, and discussions related to the formalization of mathematical logic.
+
+## How to Contact
+
+Please open a new topic at [GitHub Discussions](https://github.com/orgs/FormalizedFormalLogic/discussions) unless your message needs to be kept confidential.
+
+The reason is simple: handling similar inquiries individually (for example, requests for advice on formalizing a particular area) can easily lead to duplicated effort.
+
+If you do not have a GitHub account or you think your message should remain confidential, please send an email us using the contact information provided on the developers' page.
+
+## Language
+
+We primary use Japanese in discussion, but English is also ok.