Add license and contribution information

Author: gjbex

CONTRIBUTING.md

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+# Contributing
+
+You are very welcome to contribute to this repository.
+
+
+## How to contribute?
+
+This can be done in several ways, depending on how familiar you are with
+the GitHub workflow:
+
+  * simply send an [email]([email protected]),
+  * [create an issue](https://help.github.com/articles/creating-an-issue/)
+    on GitHub, or
+  * [fork the repository](https://help.github.com/articles/fork-a-repo/)
+    on GitHub, create a branch based on the development
+    branch, make your contribution, and submit a
+    [pull request](https://help.github.com/articles/about-pull-requests/).
+
+For pull requests, a few things should be noted:
+
+  * you may be asked to modify your contribution to fit better into
+    the context,
+  * please base them on the development branch,
+  * please ensure your branch is in sync with the upstream development
+    branch before submitting the pull request,
+  * please try to use [informative commit messages](https://chris.beams.io/posts/git-commit/)
+    (I'm not claiming that mine are shining examples, but well...), and
+  * if you want to suggest modifications to PowerPoint presentations,
+    please make a copy of the original presentation, modify it, and add it.
+
+Since from `git`'s perspective, PowerPoint files are just binary files,
+a merge can only be done using Microsoft's PowerPoint application by
+comparing two PowerPoint files.
+
+Of course, although all contributions are very much appreciated, some of
+them might not be integrated into the material.  If that should happen,
+ I'll try to justify that decision, and it can be discussed.
+
+
+## Citation and licensing
+
+For contributions in general,
+
+  * if you make a contribution, this will be mentioned in the
+    [README](README.md), unless requested otherwise;
+  * contributions will be under the same [license as this repository](LICENSE).
+
+
+## What can you contribute?
+  
+Basically, anything you like, e.g.,
+
+  * corrections of typos,
+  * bug fixes in sample code,
+  * additional sample code,
+  * extra or alternative slides in presentations,
+  * a section in a presentation,
+  * a new subject, or aspect of an existing subject.