Add description of new `use_async` to the README

[AnnMarieW]

It's great that there's now an option to generate a project that does async loading of components 🎊

It would be helpful to add a short description to the README.md in section 3 " Answer the questions about the project." so people would know what this is and if they should select 'True' or 'False'.

[nlyle01]

+1, came here to say the same thing. It would be nice to know if there are trade-offs

[danielleos]

Agree, this would be really helpful! I'm coming from a React background with little knowledge of python and plotly so would love to know what this option means.