Add contributing.md

Author: jeanp413

docs/CONTRIBUTING.md

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+# Build and Run
+
+## Prerequisites
+- [Git](https://git-scm.com/)
+- [Node.JS](https://nodejs.org/en/), >= 14.x
+- [Yarn](https://classic.yarnpkg.com/en/), follow the [installation guide](https://classic.yarnpkg.com/en/docs/install)
+- VS Code
+
+If you want explore the source code of this extension yourself, it's easy to get started. Simply follow these steps:
+
+- Clone the repository
+- Run yarn
+- Compile in the background
+    - Run yarn watch
+    - Or you can directly start this task by Command Palette -> Run Build Task
+
+To run and debug the extension, press F5.