readme: add versioning policy

Author: vladfaust

README.md

@@ -28,7 +28,9 @@ dependencies:
     version: ~> 0.8.0
 ```
 
-This shard follows [Semantic Versioning v2.0.0](http://semver.org/), so check [releases](https://github.com/onyxframework/rest/releases) and change the `version` accordingly. Please visit [github.com/crystal-lang/shards](https://github.com/crystal-lang/shards) to know more about Crystal shards.
+This shard follows [Semantic Versioning v2.0.0](http://semver.org/), so check [releases](https://github.com/onyxframework/rest/releases) and change the `version` accordingly.
+
+> Note that until Crystal is officially released, this shard would be in beta state (`0.*.*`), with every **minor** release considered breaking. For example, `0.1.0` → `0.2.0` is breaking and `0.1.0` → `0.1.1` is not.
 
 You'd also need to add a database dependency conforming to the [crystal-db](https://github.com/crystal-lang/crystal-db) interface. For example, [pg](https://github.com/will/crystal-pg):