Update README.md to include info on how to get the site running locally and push new updates

[vidhikesaria]

@yogeshkhatri1989 now that you've pushed two updates. Any chance you can write a Building locally section in readme.md that includes the steps of what is required to get a locally running site. e.g installing node, npm, yarn, webpack, adding config, yarn sync_fieldbook and running the site.

Thanks.

[yogeshkhatri1989]

Done