* Docs(download): mention other CDN
so that developers know their possible alternatives and can decide by themselves which option they want to use for their project.
None of the 3 listed options require any work from Leaflet side (as of today) for new releases, they directly pull updated from npm registry.
cdnjs uses a configuration file in their repo to know which files to fetch from npm registry:
https://github.com/cdnjs/cdnjs/blob/master/ajax/libs/leaflet/package.json
In case the `dist/` folder changes, this configuration file requires manual modification (the community may directly send a PR to do so).
* Docs(download): add disclaimer about external CDN services
in order to try avoiding questions and/or complaints in case of CDN unavailability or any other issue.
