Create node-version.md

Author: robertbrignull

docs/node-version.md

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+# Node version
+
+The CodeQL for VS Code extension defines the version of Node.js that it is intended to run with. This Node.js version is used when running most CI and unit tests.
+
+When running in production (i.e. as an extension for a VS Code application) it will use the Node.js version provided by VS Code. This can mean a different Node.js version is used by different users with different versions of VS Code.
+We should make sure the CodeQL for VS Code extension works with the Node.js version supplied by all versions of VS Code that we support.
+
+## Checking the version of Node.js supplied by VS Code
+
+You can find this info by seleting "About Visual Studio Code" from the top menu.
+
+![about-vscode](images/about-vscodeaba.png)
+
+## Updating the Node.js version
+
+The following files will need to be updated:
+- `.github/workflows/cli-test.yml` - the "node-version: '[VERSION]'" setting
+- `.github/workflows/main.yml` - all the "node-version: '[VERSION]'" settings
+- `.github/workflows/release.yml` - the "node-version: '[VERSION]'" setting
+- `extensions/ql-vscode/.nvmrc` - this will enable nvm to automatically switch to the correct node version when you're in the project folder
+- `extensions/ql-vscode/package-lock.json` - the "engines.node: '[VERSION]'" setting
+- `extensions/ql-vscode/package.json` - the "engines.node: '[VERSION]'" setting