Add info for people who forgot to upgrade their nodes

Summary: Version 0.26.6 includes the checkpoint that will cause the node to reconsider the fork block, and get them back onto the correct chain tip.

Test Plan: Ran it locally, read it and checked the link.

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12537