Proposal to add API stability marker

[namsral]

I would like to propose an additional metric to rank packages available on godoc.org. This is a proposal in succesion of #320 (Proposal for Package Ranking) and a follow-up of golang/go#17244 (comment) .

Problem

The problem I face as a Go developer is choosing the best package. Should I go for github.com/gorilla/websocket because it is popular on GitHub or golang.org/x/net/websocket because it is a subrepo of golang.org/x? Or should I pick the most imported package?

The former metrics are not objective, especially the GitHub stars, but my guess is everyone uses these metrics for lack of a better alternative.

Proposal

Add a marker to reflect the API stability of a package based on the last breaking change.

This metric won't have a significant impact on its own but in combination with ranking based on code quality, documentation, code coverage, etc, as proposed in #320 these metrics could result in a powerful automated ranking system for Go packages.

There will be side cases to deal with. One I can think of is abandoned packages which won't have breaking changes. These can be filtered out before ranking for API stability. Filtering abandoned packages is discussed in #320.

As requested by @adg in #320, this proposal was created as a seperate issue:

When we decide to embark on implementing any of these specific metrics, please create a separate issue for that particular metric so that we can nail down the design before implementation. - @adg