Define versioning and upgrade compatibility policies

[yuryu]

Is your feature request related to a problem? Please describe.
We don't have any upgrade and compatibility policies, and users have no idea what they can expect.

Describe the solution you'd like
Define a clear versioning and compatibility between versions policy. We need a statement for users, and documentation/guideline for Open Saves developers.

My current expectations are as follows

• alpha: we make breaking changes between alphas in the same release cycle, and that is expected.
• beta: we try to avoid breaking changes, but we still would if necessary. Such changes will be clearly marked in release notes.
• release: we will follow the semantic versioning scheme. major releases break things, minor releases don't.

What users could expect from the compatibility promise:

• You can upgrade the proto definition without making changes to your code base
• You can upgrade the server version while keeping all the data
• You cannot downgrade the server version once upgraded
• Newer clients are compatible with older servers (maybe limit to one or two minor releases).
• Older clients are not compatible with newer servers
• Upgrade should work from any older minor release in the same major release cycle
• Major upgrade might requires an upgrade to the latest minor release of the previous major release

Do we have anything else?