Add README for desktop download CDN service

Author: davej

packages/desktop-download-cdn/README.md

@@ -0,0 +1,60 @@
+# Desktop Download CDN
+
+A CDN service for managing and serving desktop application downloads.
+
+## Available Routes
+
+### Universal Route
+
+- `/`
+
+This route will serve the latest version of the application and use the user agent to determine the platform and architecture.
+NOTE: it's not possible to determine the architecture from the user agent on macOS, so it will default to `x64` if a universal installer is not available.
+
+### Latest Version Routes
+
+- `/:platform/:artifactName/:arch`
+- `/:platform/:artifactName`
+- `/:platform`
+
+These routes will serve the latest version of the application. The parameters are the same as above, but they will always return the most recent release.
+
+### Version-based Routes
+
+- `/versions/:appVersion/:platform/:artifactName/:arch`
+- `/versions/:appVersion/:platform/:artifactName`
+- `/versions/:appVersion/:platform`
+- `/versions/:appVersion`
+
+These routes allow downloading specific versions of the application. The parameters are:
+
+- `appVersion`: The version number of the application (e.g., "1.2.3")
+- `platform`: The operating system (e.g., "windows", "mac", "linux")
+- `artifactName`: The name of the artifact to download (e.g., "nsis", "dmg", "appimage")
+- `arch`: (Optional) The CPU architecture (e.g., "x64", "arm64")
+
+### Build-based Routes
+
+- `/builds/:buildId/:platform/:artifactName/:arch`
+- `/builds/:buildId/:platform/:artifactName`
+- `/builds/:buildId/:platform`
+- `/builds/:buildId`
+
+These routes allow downloading specific builds of the application. The parameters are:
+
+- `buildId`: The unique identifier for the build
+- Other parameters are the same as version-based routes
+
+## Examples
+
+### Download the latest Windows installer
+
+`/windows`
+
+### Download version 1.2.3 for Mac ARM
+
+`/versions/1.2.3/mac/arm64`
+
+### Download a specific build for Linux
+
+`/builds/12345/linux/appimage/x64`