refactor(docs): move from reST to Markdown (#15)

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)
+- [Diego Rodriguez](https://orcid.org/0000-0003-0649-2002)
+- [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 Authentication Kerberos5
+
+[![image](https://github.com/reanahub/reana-auth-krb5/workflows/CI/badge.svg)](https://github.com/reanahub/reana-auth-krb5/actions)
+[![image](https://img.shields.io/badge/discourse-forum-blue.svg)](https://forum.reana.io)
+[![image](https://img.shields.io/github/license/reanahub/reana-auth-krb5.svg)](https://github.com/reanahub/reana-auth-krb5/blob/master/LICENSE)
+
+## About
+
+`reana-auth-krb5` provides a container image for creating and renewing Kerberos tokens.
+The container image includes no additional logic or libraries, just the bare minimum to
+support the Kerberos operations.
+
+`reana-auth-krb5` was developed for use in the [REANA](http://www.reana.io/) reusable
+research data analysis platform.
+
+## Usage
+
+The `reana-auth-krb5` image is used internally in the REANA platform to refres the
+Kerberos token for long running jobs. The end users can ask for Kubernetes authentication
+by means of declaring `kerberos: true`, more information
+[here](http://docs.reana.io/advanced-usage/access-control/kerberos/).
+
+If you want to try it locally, a Kerberos token can be obtained via:
+
+```
+$ docker run -i -t --rm docker.io/reanahub/reana-auth-krb5:1.0.1 /bin/bash
+> kinit -k -t /path/to/keytab_file [email protected]
+> klist
+```
+
+## Configuration
+
+Running the `reana-auth-krb5` and successfully obtaining a shared token on a sidecar
+container requires additional information and inputs:
+
+- [Kerberos cache](https://web.mit.edu/kerberos/krb5-1.12/doc/basic/ccache_def.html)
+  location to be shared, configured through the `KRB5CCNAME` environment variable
+- [Kerberos configuration](https://web.mit.edu/kerberos/krb5-1.12/doc/admin/conf_files/krb5_conf.html)
+  at `/etc/krb5.conf` (overridable)
+
+## Changes
+
+Version 1.0.1 (2020-08-12)
+
+- Add CERN Kerberos configuration.
+
+Version 1.0.0 (2020-08-05)
+
+- Initial release
+
+## Development
+
+If you would like to contribute to `reana-auth-krb5` development, you can take advantage
+of the 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 [REANA](http://www.reana.io/) reusable research data analysis
+platform, please see [its documentation](http://docs.reana.io/).