fix(docs): align readme with plugins installation flow

I’m proposing a small update to this README. The goals:
- Ensure that users learn how to install through the Netlify UI
- Align it with some recent changes we’ve made to our [file-based installation instructions](https://docs.netlify.com/configure-builds/build-plugins/#file-based-installation) for Build Plugins

Author: rstavchansky

README.md

@@ -4,7 +4,9 @@ Automatically generate a sitemap for your site after it finishes building in Net
 
 ## Installation
 
-To install, add the following lines to your `netlify.toml` file:
+You can install this plugin in the Netlify UI from this [direct in-app installation link](https://app.netlify.com/plugins/@netlify/plugin-sitemap/install) or from the [Plugins directory](https://app.netlify.com/plugins).
+
+To use file-based installation, add the following lines to your `netlify.toml` file:
 
 ```toml
 [[plugins]]
@@ -13,6 +15,12 @@ package = "@netlify/plugin-sitemap"
 
 Note: The `[[plugins]]` line is required for each plugin, even if you have other plugins in your `netlify.toml` file already.
 
+To complete file-based installation, from your project's base directory, use npm, yarn, or any other Node.js package manager to add this plugin to devDependencies in `package.json`.
+
+```
+npm install -D @netlify/plugin-sitemap
+```
+
 ## Configuration
 
 By default, the plugin generates a sitemap based on the `publish` directory configured in your site build settings or `netlify.toml`.