Add Getting Started section to README.md with setup needed for contribution

Ideally I would like to have the getting started section structured as follows:

Options for replicating my home setup:
- A step-by-step setup description in the form of:
- Clone the repository (git clone --recursive https://github.com/alemorm/ucsf-radiology-getting-started.git)
- Download the latest Hugo precompiled binary release for your specific OS and processor (and instructions for how to find your OS and processor)
- Install the Hugo precompiled binary depending on the OS (Linux, Windows, MacOS)
- Alternatively, a bash/sh script that automatically downloads the appropriate release (Linux, MacOS) and installs it
Website content and generation:
- Explain the general organization of the site contents
- How to build the site and display it in a browser

alemorm / ucsf-radiology-getting-started