Create DEVELOPMENT.md

DEVELOPMENT.md

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+# Development cycle
+
+To keep up with the fast development of the Cairo language and Starknet network while maintaining a secure
+and healthy development process, **we organize our work in 3-weeks cycles**.
+These cycles consist of **milestones** and **sprints.**
+
+## 📍 Milestones
+
+A milestone is a set of [issues](https://github.com/OpenZeppelin/cairo-contracts/issues) intended to be addressed.
+We usually aim to have at least 4 milestones planned ahead (~12 weeks of work),
+enough visibility for users to set realistic expectations on when a feature will be available,
+and give us space to gauge demand.
+We can also make better decisions and prioritization when we’re mindful of the bigger picture.
+
+We organize issues in our [Github project](https://github.com/orgs/OpenZeppelin/projects/29) in the following views:
+
+- [Current milestone](https://github.com/orgs/OpenZeppelin/projects/29/views/2) (now)
+- [Next milestone](https://github.com/orgs/OpenZeppelin/projects/29/views/3): ~3 weeks from now
+- [After milestone](https://github.com/orgs/OpenZeppelin/projects/29/views/4): ~6 weeks from now
+- [Later milestone](https://github.com/orgs/OpenZeppelin/projects/29/views/5): ~9 weeks from now
+- [Backlog](https://github.com/orgs/OpenZeppelin/projects/29/views/7): issues not assigned to any milestone
+- [Good to tackle](https://github.com/orgs/OpenZeppelin/projects/29/views/10): issues not planned soon nor in progress, for contributors to take
+
+## 🏁 Sprints
+
+A sprint is a 3-week period of time in which we intend to complete a milestone.
+Some milestones might extend or shorten a little bit if issues are finished ahead or after schedule,
+while the end of a sprint marks the time to release whatever work has been finished to date,
+i.e. “[the release train departs](https://github.com/OpenZeppelin/cairo-contracts/blob/main/RELEASING.md)”.
+
+- We design milestones to take ~25 working days so we can tackle them in a single sprint
+    1. 5 working days * 3 weeks/cycle * 3 devs
+        - ⇒ 45 workdays/sprint
+    2. we estimate we spend roughly ~1/3 or our time doing reviews
+        - ⇒ 30 workdays/sprint
+    3. we apply a 1/6 reduction to account for vacations, sickness, distractions, support, etc.
+        - **⇒ 25 workdays/sprint**
+- To do so, we categorize issues by size based on a rough estimate of how many working days we expect it to take
+    - 🦔 tiny: ~0.5 days
+    - 🐇 small: ~2.5 days
+    - 🐂 medium: ~5 days
+    - 🦑 large: ~10 days