Add dev install section in contributing.md

[ajthinking]

No description provided.

[ashishjain0512]

This a welcome change

[ajthinking]

Thank you @ashishjain0512 !
May I ask you, what is the fastest way to have a play with this repo?
I see the demos, but I cant serve them. Is there a yarn start or equivalent?

[ashishjain0512]

@ajthinking You can use yarn dev to start the dev server and open local demo pages like http://localhost:9000/current.html

Also, for docs, we use the docsify, (you might need to install docsify-cli first using npm i docsify-cli -g). To run the local docs server, use docsify serve docs. Access docs pages: http://localhost:3000/#/gitgraph