add gatsby code example

Example 1: gatsby js install

npm install -g gatsby-cli


//Create new project 
gatsby new [gatsby-project-name] https://github.com/gatsbyjs/gatsby-starter-hello-world
cd [gatsby-project-name]

//initialize developer server
gatsby develop 

//create production build the project
gatsby build
gatsby serve

Example 2: create gatsby app

gatsby new my-app

Example 3: create new project gatsby

//Create new project 
gatsby new [gatsby-project-name] https://github.com/gatsbyjs/gatsby-starter-hello-world
cd [gatsby-project-name]

//initialize developer server
gatsby develop 

//create production build the project
gatsby build
gatsby serve