update git instructions for latest_branch (#3019)

* update git instructions for latest_branch

* add comments about tag

CONTRIBUTING.md

 issues with the `contributions welcome` label.
 
 ## Git Branches
+The master branch corresponds to the most recent version of the project.
+Note that this may be newer that the [latest release](https://github.com/Unity-Technologies/ml-agents/releases/tag/latest_release).
-Starting with v0.3, we adopted the
-[Gitflow Workflow](http://nvie.com/posts/a-successful-git-branching-model/).
-Consequently, the `master` branch corresponds to the latest release of
-the project, while the `develop` branch corresponds to the most recent, stable,
-version of the project.
-
-Thus, when adding to the project, **please branch off `develop`**
-and make sure that your Pull Request (PR) contains the following:
+When contributing to the project, please make sure that your Pull Request (PR) contains the following:
 
 * Detailed description of the changes performed
 * Corresponding changes to documentation, unit tests and sample environments (if

README.md

 [![docs badge](https://img.shields.io/badge/docs-reference-blue.svg)](docs/Readme.md)
 [![license badge](https://img.shields.io/badge/license-Apache--2.0-green.svg)](LICENSE)
 
+([latest release](https://github.com/Unity-Technologies/ml-agents/releases/tag/latest_release))
+([all releases](https://github.com/Unity-Technologies/ml-agents/releases))
+
 **The Unity Machine Learning Agents Toolkit** (ML-Agents) is an open-source
 Unity plugin that enables games and simulations to serve as environments for
 training intelligent agents. Agents can be trained using reinforcement learning,

docs/Installation-Windows.md

 Conda environment by typing `activate ml-agents`)_:
 
 ```sh
-git clone https://github.com/Unity-Technologies/ml-agents.git
+git clone --branch latest_release https://github.com/Unity-Technologies/ml-agents.git
+The `--branch latest_release` option will switch to the tag of the latest stable release.
+Omitting that will get the `master` branch which is potentially unstable.
-[here](https://github.com/Unity-Technologies/ml-agents/archive/master.zip).
+[here](https://github.com/Unity-Technologies/ml-agents/archive/latest_release.zip).
 
 The `UnitySDK` subdirectory contains the Unity Assets to add to your projects.
 It also contains many [example environments](Learning-Environment-Examples.md)

docs/Installation.md

 Once installed, you will want to clone the ML-Agents Toolkit GitHub repository.
 
 ```sh
-git clone https://github.com/Unity-Technologies/ml-agents.git
+git clone --branch latest_release https://github.com/Unity-Technologies/ml-agents.git
+The `--branch latest_release` option will switch to the tag of the latest stable release.
+Omitting that will get the `master` branch which is potentially unstable.
 
 The `UnitySDK` subdirectory contains the Unity Assets to add to your projects.
 It also contains many [example environments](Learning-Environment-Examples.md)

docs/Training-on-Amazon-Web-Service.md

 2. Clone the ML-Agents repo and install the required Python packages
 
     ```sh
-    git clone https://github.com/Unity-Technologies/ml-agents.git
+    git clone --branch latest_release https://github.com/Unity-Technologies/ml-agents.git
     cd ml-agents/ml-agents/
     pip3 install -e .
     ```