add notices to those who wish for a greater challenge

We add the following level-3 header to help people identify text
they may wish to skip if they wish for a greater challenge:

```
```

After that notice we add:

```
**If you want a greater challenge, don't read any further**:
just try to understand the program via the source.

If you get stuck, come back and read below for additional hints and information.
```

And sometimes follow with:

```
```

Or sometimes follow with:

```
```

Where the alt code impacts a "greater challenge", we add instead:

```
**If you want a greater challenge, don't read any further**:
just try to understand the program without looking at the Alternate code.

If you get stuck, come back and look at the Alternate code.
```

And where a given command impacts a "greater challenge", we add instead:

```
**If you want a greater challenge, don't read any further**:
just try to understand the program without running the command below.

If you get stuck, come back and run the following command:
```

Perform `make www` to perform the above and test under macOS.

2018/burton1/index.html

@@ -434,10 +434,11 @@ It adds several nice-to-have features, and allows salt-to-taste customizability.
 <li>UNIX V7 pcc (Johnson’s C compiler) on a <a href="http://simh.trailing-edge.com/">simulated PDP-11/45</a>
 (see the “<strong>SPOILER</strong> section below for further detail)</li>
 </ul>
-<div style="margin-bottom:60em;margin-top:4em">
-<strong>Spoiler below; scroll down.</strong>
-</div>
-<h3 id="spoiler">SPOILER:</h3>
+<h3 id="notice-to-those-who-wish-for-a-greater-challenge">NOTICE to those who wish for a greater challenge</h3>
+<p><strong>If you want a greater challenge, don’t read any further</strong>:
+just try to understand the program via the source.</p>
+<p>If you get stuck, come back and read below for additional hints and information.</p>
+<h3 id="how-this-entry-works">How this entry works:</h3>
 <p>This started life as a personal challenge: write the shortest hex dump utility
 in C.</p>
 <p>The self-imposed ground rules in the hunt for shortest:</p>