Document forking react-scripts instead of ejecting

Created by: thien-do

I follow @shubheksha instruction and rewrite it as a section in User Guide. I hope it can help.

Because I didn't see anyone claim to work on this yet, so I want to start a PR and we can discuss more. In case @zperrault or anyone have worked on this already, I am totally ok to close my PR.

To be honest, I know I don't have much experience writing docs but I am very active and have much time to update my PR to be better/more useful in case anyone have any suggestion.

Related issue: https://github.com/facebookincubator/create-react-app/issues/682

UPDATE: A summary of current things that need to do:

1. Fork CRA
2. Change react-scripts package name (in its package.json) (prefer scoped)
3. Mark other packages as private
4. Run npm run publish from root
5. Use it with CRA