docs: add instructions to disable built-in TS plugin

Author: ignace

docs/getting-started.md

@@ -44,3 +44,13 @@ You can install an editor extension to display Glint's diagnostics inline in you
 - Install the [VS Code Extension](https://marketplace.visualstudio.com/items?itemName=typed-ember.glint-vscode).
 
 ![A type error being shown inline for a template file in VS Code](https://user-images.githubusercontent.com/108688/111076679-995c2300-84ed-11eb-934a-3a29f21be89a.png)
+
+To get Ember/Glimmer and TypeScript working together, Glint creates a separate TS language service instance patched with Ember-specific support. To prevent invalid or duplicate diagnostics you need to disable VSCode's built-in TS language service in your project's workspace only by following these steps:
+
+1. In your project workspace, bring up the command palette with `Ctrl + Shift + P` (macOS: `Cmd + Shift + P`).
+2. Type `built` and select "Extensions: Show Built-in Extensions".
+3. Type `typescript` in the extension search box (do not remove `@builtin` prefix).
+4. Click the little gear icon of "TypeScript and JavaScript Language Features", and select "Disable (Workspace)".
+5. Reload the workspace. Glint will now take over TS language services.
+
+![Disabling built-in TS language service per workspace](https://user-images.githubusercontent.com/108688/111069039-6dc84100-84cb-11eb-8339-18a589be2ac5.png)