Mention in README/extension page why read-write access is required

[bew]

As a follow-up request about #12, I'd really appreciate if the extension description on the extension page and in the README why read-write access is required, to especially explain that write access is requested but not used anywhere 🙏.

[aidan-gibson]

This is also holding me back from installing at all. As soon as write access isn't requested I'm looking forward to trying this.