Enumerate version maintenance and version bumps

[smellsblue]

I think we should spell out in the documentation:

1. What we mean when we bump a major version
2. What we mean when we bump a minor version
3. How far back we will provide patch releases for bugs (maybe something like the last 2 minor versions of the last 2 major versions?)

I would like to follow semantic versioning, though to do so we should figure out what our API is, so we know when we are making a backwards incompatible change.

If we do so, then the biggest thing on my mind... is the new index format a backwards incompatible change or not? I think we could reasonably say that any HTTP requests happening behind the scenes are not really our API, and so I would consider it a minor update. Yet, it is still potentially a big change that introduces risk, and that might not be conveyed properly in a minor version update, so maybe that doesn't matter.

/cc @pcarranza @indirect

[indirect]

The new index format should fully backwards compatible, since it doesn't touch any existing URLs, and is made available via entirely new URLs.

FWIW, we have reached release candidate for Bundler 1.12, which has support for the new index. We've also shipped the new index in bundler-api, if you're interested in seeing a server-side implementation of it. We're planning to ship gems with the extracted client and server code, but I don't think they're done yet...

[pcarranza]

Agreement then @smellsblue ?