Add release workflow docs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: gajop

docs/releasing.md

@@ -0,0 +1,28 @@
+# Releasing SpringBoard
+
+The `SpringBoard release` workflow produces four files:
+
+```text
+SpringBoard-Core-<version>-linux-x86_64.sdz
+SpringBoard-Core-<version>-windows-x86_64.sdz
+SpringBoard-<version>-linux-x86_64.tar.gz
+SpringBoard-<version>-windows-x86_64.zip
+```
+
+There are two `.sdz` files as SpringBoard Core itself uses Rust and builds different native binaries for each system.
+
+The `.tar.gz` and `.zip` files are complete applications. They contain the engine, editor, settings, start script, and one executable named `SpringBoard` or `SpringBoard.exe`.
+
+## Workflow inputs
+
+- The compatible engine repository and release are committed in `build/distribution.json`.
+- The Actions branch selector chooses the SpringBoard source to package.
+- `publish`: off produces workflow artifacts only; on creates the tag and GitHub Release.
+
+## Versions
+
+- `stable`: `1.0.0`
+- `prerelease`: `1.0.0-beta.1`
+- `nightly`: generated as `YYYY-MM-DD-<short-sha>`
+
+Existing tags and releases are never overwritten.