Document the release policy

[dunglas]

The important part:

• stable (currently the 2.2 branch): regular bug fixes are integrated in this version
• old-stable (currently 2.1 branch): security fixes are integrated in this version, regular bug fixes are not backported in it
• development (master branch): new features target this branch

Older versions (1.x, 2.0...) are not maintained. If you still use them, you must upgrade as soon as possible.

ping @api-platform/core-team

[joelwurtz]

What about the timing of releases ? Can we expect consistent time between each releases or will this mainly depend on contribution activity ?

[dunglas]

@joelwurtz I've written this in the docs: New versions of API Platform are released **when they are ready**, on the behalf of the API Platform Core Team.

So it will mainly depend of the contribution activity for now. We may change this later. My current idea is to release a new minor version of API Platform ~1 month after the release of every minor Symfony version (so every 6 months). But I don't want to enforce this hardly for now, we may release versions more frequently if the activity level is high.