Update readme.md

Greetings from the Netlify Docs team!

I’m proposing a small update to your 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

Feel free to edit and merge as you see fit.

Author: rstavchansky

readme.md

@@ -8,11 +8,19 @@ Optimize images as part of your Netlify build process. Optimizes PNG, JPEG, GIF
 
 ## Install
 
-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-image-optim/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]]
 package = "netlify-plugin-image-optim"
 ```
 
 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-image-optim
+```