Override project URL using relative path

Right now, RSB assuming my app is served at the server root. It's hard to override it.

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Current:
we can access it via http://domain.com/ as a root. But we can't make the another project inside using sub-folder.

Future:
we can access it via http://domain.com/sub-folder/ as a root

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For your reference:

[create-react-app](https://github.com/facebookincubator/create-react-app) have an ability to change it via `package.json` -- [link](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#building-for-relative-paths)

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If you have a solution about this, maybe you should update the doc.
Thanks for creating this wonderful project!

Start by reviewing the project's URL and build configuration, then compare it with the create-react-app relative-path guidance linked in the issue. Done means the app can be served from a sub-folder such as /sub-folder/ instead of only the server root, with documentation updated if configuration is required.

Written by the indexing model from the issue text.