Define milestone-based versioning

README.md

@@ -33,6 +33,18 @@ collision-free landing position can be found nearby.
 
 The built mod is written to `build/libs/`.
 
+## Versioning
+
+Rivencraft versions use `x.y.z`:
+
+- `x` changes for a major public release.
+- `y` changes when a development milestone is completed.
+- `z` changes for iterations within the current milestone and for bug fixes.
+
+The patch component resets to zero when a milestone is completed. For example,
+milestone 2 development follows `0.1.1`, `0.1.2`, and so on; completing that
+milestone produces `0.2.0`.
+
 ## Porting approach
 
 Mystcraft Legacy targets Forge for Minecraft 1.12.2. Rivencraft therefore