user-guide: versioning semantics and compatibility policies

[jorgeorpinel]

In #896 (review) I suggested introducing the following note to https://dvc.org/doc/install

⚠️ Please try to use the same/latest DVC version on all the team member
machines. DVC is in active development and compatibility issues may arise
between significantly different versions.

Per https://groups.google.com/a/iterative.ai/forum/#!msg/support/ZBNM2dydyxU

But we then thought it's a) to "scary" and that it probably belongs in a larger topic, that of semantic versioning and compatibility issues among levels of DVC releases (see #896 (comment)). So I'm creating this ticket to discuss and decide on what to write about this.

May even impact the dev process of the core repo going forward? Cc @iterative/engineering

[jorgeorpinel]

From Ivan:

probably take Ruslan's exact language (from the Google group topic) about backward and forward compatibility.
And also, let's not scare users with is in active development phrase:

1. First of all, only projects of not much interest are not being actively developed and any mature project keeps in mind compatibility policies.
2. Second, our version number (0.80.0 currently) is already enough to scare users :)

[jorgeorpinel]

only projects of not much interest are not being actively developed

@shcheklein I'm not sure about this. A mature project probably has very unfrequent updates (e.g. a security hot-fix every few months), but it doesn't mean that it's not being maintained. It depends what we mean by "active development". Maybe there's a better phrase like "beta stage" or something else.

[skshetry]

Second, our version number (0.80.0 currently) is already enough to scare users :)

With 1.0, users will expect some kind of backward compatibility, especially when we break repo
compatibility (treeverse/dvc#3885).
I think, we should at least document this, so that we could redirect users to, politely.

And, regarding the documentation of compatibility policy, we can follow SemVer or a bit modified with:

1. Major version on repo incompatibility
2. Minor version on any features (including cli, Repo and dvc.api backward incompatibilities).
3. Patch version on bug-fixes.

[casperdcl]

also related: treeverse/dvc#3872