Introductory note/pop up on Atlas

[MLNoon]

Allow an option to add a popup box when you first load an atlas, within which we can have a welcome message, introductory text, etc. That way we can include some information about a particular atlas (linking back to the IAP main project page and vitalsigns.org on the IAP atlases, or giving more info on Trends.Earth on the maps.trends.earth page, for example).

[azvoleff]

Here is one library: https://github.com/elrumordelaluz/reactour