Add CDNJS version badge in README.md

[extend1994]

This badge will show the library version on CDNJS!

[TehShrike]

I'd lean towards adding a link to the unpkg.com distribution instead of pushing forward with CDNJS, since unpkg gets updated automatically from npm and we wouldn't have to do extra work on each release.

[extend1994]

@TehShrike CDNJS also updates automatically from npm using a mechanism called auto-update, you don't have to do extra work :)

[TehShrike]

huh. Looks like some stuff needs to be changed in package.json to enable that, I guess?

[extend1994]

Sorry for late reply. Have been busy these days 😓
Once the library has new tag/release on GitHub or new release on npm, cdnjs has a bot to detect new version and add this new version to CDN. So what cdnjs needs is tag/release rather than package.json of the lib itself :)