Update README.md

README.md

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+# IMPORTANT NOTE:
+This repository's default/`master` branch is the final update of the originally published `paper` branch.
+For the currently developed and maintained version of `gym-pybullet-drones` (using [`gymnasium`](https://gymnasium.farama.org) and [`stable-baselines3`](https://stable-baselines3.readthedocs.io/en/master/guide/algos.html) 2.0), please checkout the branch named `main`:
+```sh
+git clone -b main https://github.com/utiasDSL/gym-pybullet-drones.git
+```
+Please `git pull` frequently and feel free to open new [issues](https://github.com/utiasDSL/gym-pybullet-drones/issues) for any undesired, unexpected, or (presumably) incorrect behavior. Thanks 🙏
+
 # gym-pybullet-drones
 
 [Simple](https://en.wikipedia.org/wiki/KISS_principle) OpenAI [Gym environment](https://gym.openai.com/envs/#classic_control) based on [PyBullet](https://github.com/bulletphysics/bullet3) for multi-agent reinforcement learning with quadrotors
 
-- This repository's `master` branch is actively developed, please `git pull` frequently and feel free to open new [issues](https://github.com/utiasDSL/gym-pybullet-drones/issues) for any undesired, unexpected, or (presumably) incorrect behavior. Thanks 🙏
 - If you are interested in safe control and the companion code of ["Safe Learning in Robotics"](https://www.annualreviews.org/doi/abs/10.1146/annurev-control-042920-020211) and ["Safe Control Gym"](https://ieeexplore.ieee.org/abstract/document/9849119/), check out [`safe-control-gym`](https://github.com/utiasDSL/safe-control-gym)
 
 <img src="files/readme_images/helix.gif" alt="formation flight" width="350"> <img src="files/readme_images/helix.png" alt="control info" width="450">