Merge pull request #79 from github/clarify-n-in-readme

clarify N in Readme

README.md

@@ -31,7 +31,16 @@ import '@github/tab-container-element'
 </tab-container>
 ```
 
-If none of the tabs have `aria-selected=true`, then the first tab will be selected automatically. You can also add the `default-tab=N` attribute to avoid having to set `aria-selected=true` on the desired tab.
+If none of the tabs have `aria-selected=true`, then the first tab will be selected automatically. You can also add the `default-tab=N` attribute to avoid having to set `aria-selected=true` on the desired tab, where `N` is the 0-based tab index:
+
+```html
+<!-- The _second_ tab will be selected -->
+<tab-container default-tab="1">
+  <button type="button" id="tab-one" role="tab">Tab one</button>
+  <button type="button" id="tab-two" role="tab">Tab two</button>
+  <button type="button" id="tab-three" role="tab">Tab three</button>
+  <!-- ... -->
+</tab-container>
 
 ### Events