Add versioning policy

doc/VERSIONING.md

+rav1e follows Cargo's versioning scheme: https://doc.rust-lang.org/cargo/reference/manifest.html#the-version-field
+
+Because rav1e is not yet at version 1.0.0, all changes that break the API require a minor-version bump.
+
+The API is defined as:
+- public functions in src/api.rs
+- command line parameters to the rav1e binary