chore: update readme to add vanilla JS usage

[ayoayco]

First off, thanks for the project. It's awesome! 🙌

I recently pushed to use it on my blog, and the way I used it is with a vanilla JS webmention utility I wrote. Here's the commit: https://github.com/ayoayco/blog/commit/ff3182c946763c877f5e6134edfae847c3ff5dbb

I thought I'd submit this usage documentation as well. Let me know whether this fits your README or I should modify.

Thank you!