Publish a "snapshot version support" document

`lsm-tree` has a versioned on-disk format for snapshots. `lsm-tree` is backwards compatible with older snapshot versions. Forwards compatibility is not supported at all. `lsm-tree` committed to being backwards compatible with *at least* one snapshot version at all time, but it is not its goal to be backwards compatible indefinitely.

Users should have access to a document that describes `lsm-tree`'s procedure of adding new snapshot versions and dropping old snapshot versions. Moreover, the document should describe how users are affected by such snapshot version changes and how they should structure/modify their projects accordingly. Ideally the document would answer:

* How are users notified of new snapshot versions?
* For example: a major `lsm-tree` version bump and an entry in the changelog
* How should users structure their projects to account for future snapshot versions?
* For example: they should have tight enough bounds on `lsm-tree` versions so that all those `lsm-tree` versions produce the same snapshot version. Or, if they don't have such tight bounds, then they should implement a recovery strategy.
* How should users migrate from old snapshot versions to new ones?
* For example: open an old snapshot with a newer `lsm-tree` version, and then write it back to disk with the new format
* How are users notified of dropped old snapshot versions?
* @wenkokke suggested using a `MAJOR.MAJOR` package version bump to signal dropping support for older snapshot versions. For example, we could bump up from version `1.x` to `2.0`.
* how long should users expect versions to be supported?
* For example: at least a year?

No file or test is named; start by reviewing the snapshot-version implementation and the changelog to identify the current compatibility and release procedures. Use the issue's questions to structure the document, resolving the proposed notification, migration, support-duration, and version-bump policies with maintainers. Done means the document explains both version changes and the actions users should take.

Written by the indexing model from the issue text.