docs: update readme with installation instructions

README.md

@@ -24,6 +24,14 @@ A Visual Studio Code extension that makes life easier when working with the Gree
 
 > Not a strict requirement, but installing yaml languge support (e.g. via [YAML Extension](https://marketplace.visualstudio.com/items?itemName=redhat.vscode-yaml)) greatly improves the experience of working with IF manifest files. Without Yaml language support, some of this extension's features may not work properly (more details [here](https://github.com/andrew-woosnam/impact-framework-vscode/issues/8)).
 
+## Installation
+
+### Option 1:
+Visit the [releases page](https://github.com/andrew-woosnam/impact-framework-vscode/releases) for a .vsix file, then either double-click it or drag & drop it directly into your VS Code editor's Extensions sidebar.
+
+### Option 2:
+Clone this repository, open the project in VS Code and press F5 to build. A second sandbox instance of VS Code (called "Extension Development Host") will open with this extension installed.
+
 ## Known Issues
 
 We track these [here](https://github.com/andrew-woosnam/impact-framework-vscode/issues).