Add a README.md as per the contribution guidelines

See https://gitlab.com/pages/pages.gitlab.io/-/blob/master/CONTRIBUTING.md

README.md

+![Build Status](https://gitlab.com/pages/<project>/badges/master/build.svg)
+
+---
+
+Example Jupyter Book website using GitLab Pages.
+
+Learn more about GitLab Pages at https://pages.gitlab.io and the official
+documentation https://docs.gitlab.com/ce/user/project/pages/.
+
+---
+
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+**Table of Contents**  *generated with [DocToc](https://github.com/thlorenz/doctoc)*
+
+- [GitLab CI](#gitlab-ci)
+- [Building locally](#building-locally)
+- [GitLab User or Group Pages](#gitlab-user-or-group-pages)
+- [Did you fork this project?](#did-you-fork-this-project)
+- [Troubleshooting](#troubleshooting)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+
+## GitLab CI
+
+This project's static Pages are built by [GitLab CI][ci], following the steps
+defined in [`.gitlab-ci.yml`](.gitlab-ci.yml):
+
+```bash
+# Full project: https://gitlab.com/pages/jupyter-book-example
+
+stages:
+  - build
+  - deploy
+
+jupyter-build:
+  stage: build
+  image: python:slim
+  script:
+    - pip install -U jupyter-book
+    - jupyter-book clean .
+    - jupyter-book build .
+  artifacts:
+    paths:
+      - _build/
+
+pages:
+  stage: deploy
+  image: busybox:latest
+  script:
+    - mv _build/html public
+  artifacts:
+    paths:
+      - public
+  rules:
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
+```
+
+## Building locally
+
+To work locally with this project, you'll have to follow the steps below:
+
+1. [Install JupyterLab](https://jupyter.org/install) JupyterLab
+1. [Install Jupyter Book](https://jupyterbook.org/start/overview.html#install-jupyter-book)
+1. Fork, clone or download this project
+1. `cd jupyter-book-example` -- the repository contains already an example of a
+   book's source files, as described at [Jupyter Book > Create your book’s source files](https://jupyterbook.org/start/create.html#create-your-books-source-files)
+1. Work on it... see for example [Jupyter Book > Structure and organize your content](https://jupyterbook.org/basics/organize.html)
+1. Make a commit
+1. See the book published online at 
+
+Read more at Jupyter Book's [Introduction](https://jupyterbook.org/intro.html).
+
+## GitLab User or Group Pages
+
+To use this project as your user/group website, you will need one additional
+step: just rename your project to `namespace.gitlab.io`, where `namespace` is
+your `username` or `groupname`. This can be done by navigating to your
+project's **Settings**.
+
+Read more about [user/group Pages][userpages] and [project Pages][projpages].
+
+## Did you fork this project?
+
+If you forked this project for your own use, please go to your project's
+**Settings** and remove the forking relationship, which won't be necessary
+unless you want to contribute back to the upstream project.
+
+## Troubleshooting
+
+1. CSS is missing! That means two things:
+
+    Either that you have wrongly set up the CSS URL in your templates, or
+    your static generator has a configuration option that needs to be explicitly
+    set in order to serve static assets under a relative URL.
+
+[ci]: https://about.gitlab.com/gitlab-ci/
+[jupyter-book]: http://jupyterbook.org
+[install]: https://jupyterbook.org/start/overview.html#install-jupyter-book
+[documentation]: https://jupyterbook.org/intro.html
+
+----
+
+Forked from @NikosAlexandris -- Thank you @stingrayza