docs: Directly link to GitHub project page from docs website (#719)

* Add a badge to the README that links to the GitHub project

Author: matthewfeickert

README.md

@@ -4,6 +4,7 @@
 
 # pure-python fitting/limit-setting/interval estimation HistFactory-style
 
+[![GitHub Project](https://img.shields.io/badge/GitHub--blue?style=social&logo=GitHub)](https://github.com/scikit-hep/pyhf)
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.1169739.svg)](https://doi.org/10.5281/zenodo.1169739)
 
 [![GitHub Actions Status: CI](https://github.com/scikit-hep/pyhf/workflows/CI/CD/badge.svg)](https://github.com/scikit-hep/pyhf/actions?query=workflow%3ACI%2FCD+branch%3Amaster)