chore(gatsby-plugin-mdx): Add comment why rehype-slug is required

[justinyaodu]

Documentation

I added a comment describing why rehype-slug is required in this example config, which illustrates the use of rehype plugins with gatsby-plugin-mdx. I was referencing this example earlier today, but I missed the require line for rehype-slug, which caused rehype-autolink-headings to do nothing. Apparently, this has happened to someone else as well; see StackOverflow.

I realize that this example config is perhaps intended to illustrate the use of rehype plugins in general, not rehype-autolink-headers specifically. I thought I'd put it out here though, and let you all decide 🙂

[LekoArts]

Thanks!