[e.cash] Add instructions on how to upgrade a node

Summary: Help people who need instructions for how to actually upgrade their node

Test Plan:
Run it locally to see that it looks good.
Follow the instructions to check that they work.

Reviewers: johnkuney, Fabien, #bitcoin_abc

Reviewed By: johnkuney, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14630