adding markdown version of the contributing.rst file (#2086)

Author: rcap107

.github/CONTRIBUTING.md

@@ -0,0 +1,17 @@
+# Contributing guide
+
+First off, thank you for taking the time to contribute!
+
+## Have a question?
+
+If you have any questions, feel free to reach out:
+
+- Join our community on [Discord](https://discord.gg/ABaPnm7fDC) for
+  general chat and Q&A.
+- Alternatively, you can [start a discussion on
+  GitHub](https://github.com/skrub-data/skrub/discussions).
+
+
+## How can I contribute?
+
+You can find the [contributing guide](https://skrub-data.org/stable/CONTRIBUTING.html) on the website.