how to add font icon in gatsby code example

Example: how to add font icon in gatsby

npm i -S @fortawesome/fontawesome-svg-core
npm i -S @fortawesome/free-solid-svg-icons
npm i -S @fortawesome/react-fontawesome

# yarn
yarn add @fortawesome/fontawesome-svg-core
yarn add @fortawesome/free-solid-svg-icons
yarn add @fortawesome/react-fontawesome