docs(readme): add examples for optional tag

Author: scottohara

README.md

@@ -241,7 +241,68 @@ module.exports = {
 
 If the tag name is not specified it will process all the tags.
 
-> ⚠ You must specify the tag name if using `...` to extend the default sources list
+**Note:** source with a `tag` option takes precedence over source without.
+
+For example, to process `data-src` attributes on _all_ tags, omit the tag name:
+
+**webpack.config.js**
+
+```js
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.html$/i,
+        loader: "html-loader",
+        options: {
+          sources: {
+            list: [
+              {
+                attribute: "data-src",
+                type: "src",
+              },
+            ],
+          },
+        },
+      },
+    ],
+  },
+};
+```
+
+> ⚠ You **must** specify a tag name if using `...` to extend attributes for tags already in the default sources list
+
+For example, to extend the default source list so that it also processes `data-src` attributes on _all_ tags, you might be tempted to do this:
+
+```js
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.html$/i,
+        loader: "html-loader",
+        options: {
+          sources: {
+            list: [
+              // All default supported tags and attributes
+              "...",
+              {
+                attribute: "data-src",
+                type: "src",
+              },
+            ],
+          },
+        },
+      },
+    ],
+  },
+};
+```
+
+However this will only process `data-src` attributes on tags that _aren't in the default list_:
+
+- `<p data-src="..">` will be processed, as `p` is not in the default sources list
+- `<img data-src="..">` won't be processed, as `img` is already in the default sources list
 
 > You can use your custom filter to specify html elements to be processed.
 
@@ -322,8 +383,6 @@ module.exports = {
 };
 ```
 
-**Note:** source with a `tag` option takes precedence over source without.
-
 Filter can be used to disable default sources.
 
 For example: