add button that links to repo in docs (#35)

README.md

@@ -3,6 +3,7 @@
 [![License: GPL v3](https://img.shields.io/badge/License-GPL%20v3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0.html)
 [![Github Actions Status](https://github.com/open-atmos/jupyter-utils/workflows/Pylint/badge.svg?branch=main)](https://github.com/open-atmos/jupyter-utils/actions)
 [![PyPI version](https://badge.fury.io/py/open-atmos-jupyter-utils.svg)](https://pypi.org/project/open-atmos-jupyter-utils)
+[![Github repo](https://img.shields.io/badge/jupyter--utils-code_repository-gold?logo=github)](https://github.com/open-atmos/jupyter-utils)
 
 Utility routines used in Jupyter notebooks in [PySDM](https://github.com/open-atmos/PySDM), [PyMPDATA](https://github.com/open-atmos/PyMPDATA) and [PyPartMC](https://github.com/open-atmos/PyPartMC) projects:
 - [``show_plot()``](https://open-atmos.github.io/jupyter-utils/open_atmos_jupyter_utils/show_plot.html) - a drop-in replacement for matplotlib's show() displaying the figure inline using vector graphics (svg) by default and offering a download-as-pdf-or-svg widget just below (on Colab the widget triggers Google Drive download)