Re: [w3c/manifest] Merges the explainer into the readme.md (PR #1246)

@Copilot commented on this pull request.

## Pull request overview

Merges the Web App Manifest explainer into the main README for easier access.

**Changes:**
- Moves explainer content into `README.md`.
- Expands internationalization, description, and color guidance.
- Removes the standalone explainer.

### Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

| File | Description |
| ---- | ----------- |
| README.md | Adds the explainer and expanded guidance. |
| explainer.md | Removes the superseded standalone document. |







---

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> +## Icons
+
+There needs to be an icon associated with a web app, rather than the browser’s icon. To handle this, the manifest has an icons property. This takes a list of icons and their sizes, and format. Having these optional properties makes icon selection really powerful, because it provides a responsive image solution for icons – which can help avoid unnecessary downloads and helps to make sure your icons always look great across a range of devices and screen densities.
+
+```JS
+{
+  "icons": [{
+    "src": "icon/lowres",
+    "sizes": "64x64",
+    "type": "image/webp"
+  }, {
+    "src": "icon/hd_small",
+    "sizes": "64x64"
+  }, {
+    "src": "icon/hd_hi",
+    "sizes": "128x128",

This trailing comma makes the manifest example invalid JSON, so copying it into a manifest will fail JSON parsing. Remove the comma after the final `sizes` value.

> +## How can I detect if the user "installed" my app?
+The spec provides a way for you to detect when the user installs your apps by registering for "appinstalled" events.
+
+```JS
+function handleInstalled(ev) {
+  const date = new Date(ev.timeStamp / 1000);
+  console.log(`Yay! Our app got installed at ${date.toTimeString()}`);
+}
+
+// Using the event handler IDL attribute
+window.onappinstalled = handleInstalled;
+
+// Using .addEventListener()
+window.addEventListener("appinstalled", handleInstalled);
+```
+
+However, for privacy reasons, you can't directly detect if your application is installed - only if the manifest is being used with your web application.

This section says the specification defines `appinstalled`, but that event was removed from this specification (the specification changelog records “Remove beforeinstallprompt and appinstalled events”). Keeping this text in the newly canonical README incorrectly presents a non-standard API as part of Web App Manifest; replace it with the current limitation.

> +This repository is the home of the :star: **[Web App Manifest](https://www.w3.org/TR/appmanifest/)** :star: specification being worked on by
+the [Web Applications Working Group](https://www.w3.org/2019/webapps/).
+
 ## Useful links

These lines duplicate the repository introduction already present at the top of the README. Remove the repeated introduction so the merged explainer flows directly from attribution to useful links.

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