add versioning policy

docs/versioning.md

@@ -0,0 +1,32 @@
+# Versioning & Development Cycle
+
+The versioning policy follows the [Semantic Versioning](https://semver.org/).
+Since Tutu is an app for the end user, we redefine the some words in the policy:
+
+- API changes: the app is no longer available on certain platforms.
+
+## Development Cycle
+
+Dependency Freeze -> Development -> Release
+
+### Dependency Freeze
+
+This step is for:
+
+- Update dependencies
+- Prepare the new version (like, bump the version number).
+
+New dependencies should not be added in this step.
+
+### Development
+
+In this step, dependencies can only be updated if it's required to fix bugs.
+
+New dependencies should be added as their use, in this step.
+
+### Release
+
+The version is released to production in this step.
+
+Before the next development step, new versions can still be released to
+fix bugs.