Add usage example for useContext. (#2414)

* Add usage example for useContext.

The intent is to show where the hook fits in to the usual Context usage.

* Remove unnecessary comment in code block.

content/docs/hooks-reference.md

@@ -196,6 +196,50 @@ A component calling `useContext` will always re-render when the context value ch
 >
 >`useContext(MyContext)` only lets you *read* the context and subscribe to its changes. You still need a `<MyContext.Provider>` above in the tree to *provide* the value for this context.
 
+**Putting it together with Context.Provider**
+```js{31-36}
+const themes = {
+  light: {
+    foreground: "#000000",
+    background: "#eeeeee"
+  },
+  dark: {
+    foreground: "#ffffff",
+    background: "#222222"
+  }
+};
+
+const ThemeContext = React.createContext(themes.light);
+
+function App() {
+  return (
+    <ThemeContext.Provider value={themes.dark}>
+      <Toolbar />
+    </ThemeContext.Provider>
+  );
+}
+
+function Toolbar(props) {
+  return (
+    <div>
+      <ThemedButton />
+    </div>
+  );
+}
+
+function ThemedButton() {
+  const theme = useContext(ThemeContext);
+
+  return (
+    <button style={{ background: theme.background, color: theme.foreground }}>
+      I am styled by theme context!
+    </button>
+  );
+}
+```
+This example is modified for hooks from a previous example in the [Context Advanced Guide](/docs/context.html), where you can find more information about when and how to use Context.
+
+
 ## Additional Hooks {#additional-hooks}
 
 The following Hooks are either variants of the basic ones from the previous section, or only needed for specific edge cases. Don't stress about learning them up front.