[DOC] Explain how to include in Shiny apps or an RMarkdown file

[tbouffard]

For example, we can write something like in http://christophergandrud.github.io/networkD3/

We can also include some live rendering examples in the documentation via RMarkdown

See also #23 (comment) for a working example with our package

[tbouffard]

For this one, we need to generate the documentation in the HTML format or provided examples with the R format, so this will be managed after the MVP.
✔️ The shinny apps integration will be first documented in the README of the repository. Done in #42.