Meta: Update Explainer Introduction to include "readwrite-unsafe" example.

[nathanmemmott]

Updates explainer introduction so that "readwrite-unsafe" is included in the example.

[a-sully]

Looks good, though I'll copy this note from #67 (review):

please read https://github.com/whatwg/meta/blob/main/MAINTAINERS.md and in particular https://github.com/whatwg/meta/blob/main/COMMITTING.md for commit message guidelines. We try to stick to certain rules which in turn influences what ends up on Twitter and such and also ensures our commit logs are readable/scannable across standards.

So in this case, please prefix the title with Meta: (since this isn't updating the spec itself) and make sure to squash and merge according to those guidelines. Lemme know if you have any questions :)