Adding README to guide repo organization and contributions.

README.md

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+OpenMC Notebooks
+----------------
+
+This repository contains a set of example Jupyter notebooks demonstrating
+various capabilities in OpenMC from model building to advanced features like
+depletion/activation and CAD geometry.
+
+Notebook versioning
+-------------------
+
+Notebooks contained in the `main` branch of this repository should work with the
+latest version of OpenMC. Note, `main` **is not the default branch of this
+repository** and must be checked out after cloning the repo.
+
+Previous versions of the notebooks will be tagged with the corresponding OpenMC
+version with which they work. These tags can be used to checkout notebooks
+ensured to be compatible with the corresponding release of OpenMC.
+
+Continuous development of these notebooks occurs in the `develop` branch, the
+default branch of this repository. We ask that any updates or contributions to
+the repository occur based on this branch.