📖 Generator is "pre-release" in doc (#179)

* Pre-release in doc

* update

Author: laurentsimon

README.md

@@ -22,6 +22,7 @@ To generate SLSA provenance for your Golang project, follow [internal/builders/g
 ## Other projects
 
 To generate SLSA provenance for other programming languages, follow [internal/builders/generic/README.md](internal/builders/generic/README.md).
+This is a pre-release only and we will have the official release in July 2022.
 
 ## Provenance format