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.

1993/plummer/README.md

@@ -125,7 +125,16 @@ compilers) complain that there is no control path leading to a return from
 `main()`.  At 125 characters, it still fits on a single line (for those of us
 who don't mind ruining our eyes with 132 column screens).
 
-### Obfuscation:
+
+### NOTICE to those who wish for a greater challenge
+
+**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.
+
+
+### How this entry works:
 
 The first statement points the base of `l` (the argument vector) to
 be the first argument by incrementing it.  It then assigns `O` to

1993/plummer/index.html

@@ -481,7 +481,11 @@ return causes the line just written to be overwritten. <code>lint</code> (and mo
 compilers) complain that there is no control path leading to a return from
 <code>main()</code>. At 125 characters, it still fits on a single line (for those of us
 who don’t mind ruining our eyes with 132 column screens).</p>
-<h3 id="obfuscation">Obfuscation:</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>The first statement points the base of <code>l</code> (the argument vector) to
 be the first argument by incrementing it. It then assigns <code>O</code> to
 point to the character before the first character of the second