refactor(docs): move from reST to Markdown (#24)

Convert docs from reStructuredText to Markdown so that the changelog
file is compatible with Release Please.

AUTHORS.md

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+# Authors
+
+The list of contributors in alphabetical order:
+
+- [Audrius Mecionis](https://orcid.org/0000-0002-3759-1663)
+- [Daniel Prelipcean](https://orcid.org/0000-0002-4855-194X)
+- [Lukas Heinrich](https://orcid.org/0000-0002-4048-7584)
+- [Marco Donadoni](https://orcid.org/0000-0003-2922-5505)
+- [Tibor Simko](https://orcid.org/0000-0001-7202-5803)

README.md

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+# REANA Environment ROOT6
+
+[![image](https://github.com/reanahub/reana-env-root6/workflows/CI/badge.svg)](https://github.com/reanahub/reana-env-root6/actions)
+[![image](https://img.shields.io/badge/discourse-forum-blue.svg)](https://forum.reana.io)
+[![image](https://img.shields.io/github/license/reanahub/reana-env-root6.svg)](https://github.com/reanahub/reana-env-root6/blob/master/LICENSE)
+
+## About
+
+`reana-env-root6` provides a container image with encapsulated runtime execution
+environment for [ROOT6](https://root.cern.ch/) based data analyses.
+
+`reana-env-root6` was developed for use in the [REANA](http://www.reana.io/) reusable and
+reproducible research data analysis platform.
+
+## Usage
+
+You can use `reana-env-root6` provided "as is" and simply mount your own ROOT macros to
+the running container.
+
+Alternatively, you can also use `reana-env-root6` as a base image for containerising your
+own ROOT6-based analyses. Just start your `Dockerfile` from this base image and add your
+own custom ROOT code on top:
+
+```
+FROM docker.io/reanahub/reana-env-root6
+ADD my-macro.C /code/
+[...]
+```
+
+Here are several complete examples on how to use `reana-env-root6` environment in your
+analyses:
+
+- [reana-demo-root6-roofit](https://github.com/reanahub/reana-demo-root6-roofit)
+- [reana-demo-bsm-search](https://github.com/reanahub/reana-demo-bsm-search)
+- [reana-demo-lhcb-d2pimumu](https://github.com/reanahub/reana-demo-lhcb-d2pimumu)
+
+## Development
+
+If you would like to contribute to `reana-env-root6` development, you can take advantage
+of provided `Makefile`:
+
+```
+$ make build  # build a new version of the container image
+$ make test   # test the built image
+$ make push   # push it to Docker Hub
+```
+
+## More information
+
+For more information about the REANA reusable analysis platform, please see the
+[REANA project](http://www.reana.io) home page and the general
+[REANA documentation](http://reana.readthedocs.io/) pages.