Explain schema versions

Author: mgorny

peps/pep-0825.rst

@@ -185,6 +185,10 @@ version number is stored as part of the schema_ URL. Whenever the format
 changes, the version number must be incremented. Tools MUST assume that
 all variant wheels using an unknown format version are unsupported.
 
+The version numbers starting with zero are reserved for drafts and MUST
+NOT be used in production. Once the proposal is complete, the latest
+draft will be promoted to version ``1.0.0``.
+
 The top-level keys are described in the subsequent sections.