Add release instructions for installer-downloader

Author: dlon

installer-downloader/README.md

@@ -0,0 +1,44 @@
+# Installer downloader
+
+This directory contains the source code for a minimal downloader/installer for the Mullvad VPN app.
+
+## Making a release
+
+This section describes how to build and release the downloader itself. Note that this is totally
+unrelated to releasing new versions of the Mullvad VPN app.
+
+The installer is built locally and then uploaded to `app-build-linux`, which runs
+`buildserver-upload.sh`. `buildserver-upload.sh` will take care of GPG signing it and publishing to
+CDNs.
+
+`<version>` below will be used below to refer to the version that is being released.
+
+### Prerequisites
+
+1. Install Rust if you do not have it. It can be obtained from https://rustup.rs/
+
+1. You'll typically need a Windows and a macOS machine, since the installer must be built on both.
+
+1. Set up macOS and Windows for signing:
+
+    * macOS: `CSC_LINK` must point to the path of a `.p12` certificate used for code signing.
+
+    * Windows: `CERT_HASH` must be set to the fingerprint of the signing certificate to use. This
+               must be present in the certificate store.
+
+    See the [app release instructions](../Release.md) for more details on this.
+
+1. Set up `~/.ssh/config` for `app-build-linux`. Build artifacts will be `sftp`'d here.
+
+### Updating the version
+
+1. Bump the version in `Cargo.toml`.
+
+1. Create a signed tag called `desktop/installer-downloader/<version>` and `git push` the tag.
+
+### Build and upload
+
+Perform this step on both macOS and Windows:
+
+1. Run `./build.sh --upload --sign`. This will build a release build of the installer and upload it
+   to `app-build-linux:upload/installer-downloader/<version>`