Add information specifically to FieldTypes page

Author: rickbutterfield

16/umbraco-forms/developer/extending/adding-a-fieldtype.md

@@ -172,6 +172,18 @@ With Forms 14+, aspects of the presentation and functionality of the custom fiel
 
 To create custom backoffice components for Umbraco 14, it's recommended to use a front-end build setup using Vite, TypeScript, and Lit. For more information, see the [Extension with Vite, TypeScript, and Lit](https://app.gitbook.com/s/G1Byxw7XfiZAj8zDMCTD/tutorials/creating-your-first-extension#extension-with-vite-typescript-and-lit) article.
 
+The examples here are using the `@umbraco-forms/backoffice` package to get access to Forms-specific types and contexts. It is recommended to install this package as a development dependency in your project.
+
+{% hint style="warning" %}
+Ensure that you install the version of the Backoffice package compatible with your Umbraco Forms installation. You can find the appropriate version on the [@umbraco-forms/backoffice npm page](https://www.npmjs.com/package/@umbraco-forms/backoffice).
+{% endhint %}
+
+```bash
+npm install -D @umbraco-forms/[email protected]
+```
+
+This will add a package to your devDependencies containing the TypeScript definitions for Umbraco Forms.
+
 To display a name and description on a custom field, you need to register a JavaScript file as shown in the [Localization](https://app.gitbook.com/s/7MBVdnTbFiAgWuRsHpNS/customizing/extending-overview/extension-types/localization) article.
 
 ### Field Preview