By choosing a license for your project (here a public domain dedication), you effectively make a promise to your users about not only your own code but also all the dependencies for your code and the licenses of these dependencies.
In making a promise, you should be clear about your policy for these dependencies: are they all under the same terms (e.g. CC0) or do they use other licenses? If they do, they you should disclose this upfront as the combination of these deps may impose additional requirements on your re-users.
For instance if you depend on leaflet, its main license is a BSD, but it has plugins that you may use that themselves under other licenses and so on (here at least some MIT and Apache license at the first level of deps). And this all the way for the full dependency tree.
With the mere inclusion of a CDN URL you are also redistributing IMHO (the fact this is transferred from a CDN is irrelevant IMHO there, as you are the one triggering the effective reuse) and with redistribution you have obligations (in this case and per the license terms you would need to include somewhere at least the text of the leaftlet license and this text is inconveniently not included in the CDN itself).
So I would be clear in my README that there are other licensed components that are deps and what are the licenses. And I would include in my app proper some about page that lists all these and includes the corresponding license texts (and comply with anything else in terms of attribution and/or redistribution)
(side note: to help find out which licenses are used and what is the tree of deps you could check out my scancode)