Add README content

README.md

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 # Pac Macro Mapview
 
-Website to display the Pac Macro players' locations.
+Web application to display the Pac Macro game.
+
+The web application is **not yet available** as a website due to issues with HTTP content served over an HTTPS connection.
+
+To run the web application, download the project and open `index.html` in a browser. An [Electron](https://electronjs.org) application is also available as an option - [download the correct executable for your operating system](https://github.com/pacmacro/pacmacro.github.io/releases).
+
+![Screenshot](readme-img/screenshot.png)
+
+## Electron Setup
+
+To start the Electron application locally from the project files, install Electron as a development dependency in the project:
+```
+npm install --save-dev electron
+```
+
+Then start the application:
+```
+npm start
+```
+
+To build the executables, run:
+```
+./build/release.py
+```
+
+The distributables will be in `build/` as zip files.