Add docs how to release

Author: jeanp413

docs/releasing.md

@@ -0,0 +1,9 @@
+# How to release
+
+1. Edit version in [package.json](https://github.com/gitpod-io/gitpod-vscode-desktop/blob/master/package.json)
+    - Update version of the extension - this is usually the minor version. **Until the marketplace supports semantic versioning, the minor version should always be an even number. Odd numbers are reserved for the pre-release version of the extension.**
+    - (If necessary) Update vscode engine version
+
+2. If the minor version was increased, run the Nightly action to ensure a new pre-release version with the increased version number is released
+
+3. Run the Release action.