Feature request: prominent notification of breaking changes when updating

[jrial]

Please consider displaying warnings, preferably with a prompt to continue or abort, when an update will break things.

Motivation:

In pyenv/pyenv#2009 both me and the OP were facing the issue of pyenv suddenly breaking after the update to 2.0. Turns out the installation instructions had changed, and we needed to pass an extra flag somewhere. This was apparently documented in the README, which one is supposed to read during installation. However, this wasn't an installation, it was an update from the commandline, which shouldn't require a visit to the github repo to check the README again.

Many other applications already display warnings when an update requires manual intervention or special attention due to incompatible changes. Debian's package management system is a great example of that. I believe it would be a good idea to keep a changelog of breaking changes, linked to the release numbers, and have pyenv update display the relevant ones when updating between versions.