docs: add readme file to packages/d3fc
docs: add readme file to packages/d3fc

When using lerna monorepo you need to provide README.md for all relevant folders inside `/packages`.
For example: https://github.com/facebook/create-react-app/tree/master/packages/create-react-app
results
https://www.npmjs.com/package/create-react-app
ColinEberhardt/d3fc,d3fc/d3fc,d3fc/d3fc,ColinEberhardt/d3fc,ScottLogic/d3fc,d3fc/d3fc,ScottLogic/d3fc,jleft/d3-financial-components,jleft/d3-financial-components,ColinEberhardt/d3fc,d3fc/d3fc,ScottLogic/d3fc,jleft/d3-financial-components