Describe Cozystack release policy

[NickVolynkin]

User-facing version of https://github.com/cozystack/cozystack/blob/main/docs/release.md

Follow-up to cozystack/cozystack#127

[lllamnyp]

• Do we want to release every new feature in main for a minor release or do we cherry-pick something?
• How do we navigate the commits in main to cherry-pick what we need, not forget important commits and not cherry-pick something that we don't want to release yet?
• etc