README should mention that manifest can (and should) be generated by a tool

[colin-valentini]

The README.md does not mention that the custom-elements.json manifest file can (and I'm assuming should) be generated by a tool, and not created by hand. I only found that information at this dev.to article, which points to @custom-elements-manifest/analyzer.

I think adding a brief statement about generation/curation is necessary since Google searching for "custom-elements.json" yields this repository as the top result. Without this statement, you have to a do extra searching to find out that you can generate/curate the manifest, and how.