Update readme: How to fork, build, tag and install

[interactivellama]

Update https://github.com/salesforce/design-system-react/blob/master/docs/release.md#release-to-your-own-fork

You can't override scope such as @salesforce for one thing. A fork will create a design-system-react folder in your node_modules folder.

Webpack module resolve might be a good option.