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

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](https://dev.to/open-wc/introducing-custom-elements-manifest-gkk), which points to [`@custom-elements-manifest/analyzer`](https://github.com/open-wc/custom-elements-manifest/tree/master/packages/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.

Review README.md alongside the linked @custom-elements-manifest/analyzer information. Add a brief note explaining that custom-elements.json can and should be generated or curated with a tool rather than created by hand, including how to find the analyzer. Done means the README clearly communicates this to readers searching for the manifest format.

Written by the indexing model from the issue text.