Create CONTRIBUTING.md

Add simple contribution guidelines.

CONTRIBUTING.md

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+# How to contribute
+
+Please keep all code, comments, issues and pull requests in English, otherwise they will be **removed**.
+
+## Found a bug?
+
+- First, check there are no already open issues for the bug. See [here](https://github.com/ntop/nDPI/issues).
+- If there is not open issue addressing the bug, [open a new one](https://github.com/ntop/nDPI/issues/new).
+- Make sure to include a **descriptive and clear title and description**, including the **version of nDPI** being used.
+- Try to include steps to reproduce the bug.
+- If applicable, try to provide a stacktrace.
+- Providing a simple test case which shows the bug would help resolve the issue faster.
+
+## Written a new protocol dissector?
+
+- Ensure you have followed the guide in the [README](https://github.com/ntop/nDPI#how-to-add-a-new-protocol-dissector).
+- Open a new pull request with a **clear, descriptive** title.
+- Ensure the pull request passes CI testing. Any pull request which fails CI will be **ignored**.
+- If possible, add a new test for the new protocol. See [existing tests](https://github.com/ntop/nDPI/tree/dev/tests) for examples.
+
+## Written a fix for a bug?
+
+- Open a new pull request with a **clear, descriptive** title.
+- If there is an open issue for the bug, ensure the description of the pull request contains `Fixes #issue_number`.
+- Ensure the pull request passes CI testing. Any pull request which fails CI will be **ignored**.
+
+Finally, thank you for your contributions.