Improve local setup prerequisite instructions

[jrumpe]

Issue description

Setting up the project locally requires one to have installed Python. This should be included in a prerequisite section to make it easier for beginners to setup the project.

Steps to reproduce

• Install Node.js
• Install Gatsby CLI globally - npm install --global gatsby-cli
• Create a new Gatsby site: gatsby new demo https://github.com/myusername/devblog

Screenshot error