Proposal: Add ES modules footnote to the main page

### Context

Continuing the somewhat stale discussion from #316, there are lots of people who end up on the compat-table and cmd+f "module" or "import" or "export" who are all trying to get to the same thing (myself included).

See #1550 and #1224 for some examples -- these are not duplicate issues but rather sincere feedback from users whose expectation is that this type of table at least mentions these language features. If a handful of people have created issues saying they are confused, then in reality this is affecting a lot more users who won't go through the process of creating an issue.

Commonjs vs ES modules are one of the most confusing parts for entry and intermediate JS devs and I'd really love for `compat-table` to help with understanding their relative support across the ecosystem.

**#1053 sounds like a great solution**, but it will take a lot of work to get there, so I'd like to propose a really simple workaround for now.

### Proposal

I'd like to add a user-friendly footnote at the bottom of the homepage that briefly explains why ES modules are not listed here with links to their respective `caniuse` / `mdn` pages.

The key here is to mention each of these keywords (module, import, export) that people expect to find and doing so in a way that would require minimal effort on the maintainer's part.

I'd be happy to champion this PR fwiw.

And thanks for the wonderful project 🙏 I've definitely referenced `compat-table` and `node.cool` many times over the past few years.