General: Add contribution documentation.

Author: Christoph Daniel Schulze

CONTRIBUTING.md

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+# Contributing to Eclipse Layout Kernel
+
+Thanks for your interest in this project.
+
+## Project description
+
+Visual languages are popular for visualizing concepts and systems. However, just
+being graphical in nature does not automatically make a diagram easy to
+understand. Its usefulness much rather depends on the placement of its elements,
+that is: on its layout. Getting the layout right is labour intensive and time
+consuming and can be made a lot easier by making algorithms do the work. The
+Eclipse Layout Kernel (ELK) provides such layout algorithms, as well as an
+infrastructure to connect them to diagram editors.
+
+* https://www.eclipse.org/elk/
+
+## Developer resources
+
+Information regarding source code management, builds, and more.
+
+* https://www.eclipse.org/elk/documentation/contributors.html
+
+The project maintains the following source code repositories
+
+* https://github.com/eclipse/elk
+* https://github.com/eclipse/elk-models
+
+## Eclipse Contributor Agreement
+
+Before your contribution can be accepted by the project team contributors must
+electronically sign the Eclipse Contributor Agreement (ECA).
+
+* http://www.eclipse.org/legal/ECA.php
+
+Commits that are provided by non-committers must have a Signed-off-by field in
+the footer indicating that the author is aware of the terms by which the
+contribution has been provided to the project. The non-committer must
+additionally have an Eclipse Foundation account and must have a signed Eclipse
+Contributor Agreement (ECA) on file.
+
+For more information, please see the Eclipse Committer Handbook:
+https://www.eclipse.org/projects/handbook/#resources-commit
+
+## Contact
+
+Contact the project developers via the project's "dev" list.
+
+* https://dev.eclipse.org/mailman/listinfo/elk-dev