Add contributing section

README.md

@@ -193,6 +193,21 @@ The command-line interface stays open in the background,
 <img src="static/cli_jupyter_lab.png" width="50%" align="center">
 
 
+## Contributing
+
+Feedback **is highly encouraged** and will be incorporated.
+Open an issue in the [issues tracker <img height="12" style="display: inline-block" src="static/link/to_gh.png">](https://github.com/webartifex/intro-to-python/issues)
+    or initiate a [pull request <img height="12" style="display: inline-block" src="static/link/to_gh.png">](https://help.github.com/en/articles/about-pull-requests)
+    if you are familiar with the concept.
+Simple issues that *anyone* can **help fix** are, for example,
+    **spelling mistakes** or **broken links**.
+If you feel that some topic is missing entirely, you may also mention that.
+The materials here are considered a **permanent work-in-progress**.
+
+A "Show HN" post about this course was made on [Hacker News <img height="12" style="display: inline-block" src="static/link/to_hn.png">](https://news.ycombinator.com/item?id=22669084)
+    and some ideas for improvement were discussed there.
+
+
 ## About the Author
 
 Alexander Hess is a PhD student