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.

2015/endoh3/README.md

@@ -64,7 +64,15 @@ the source code from all angles..
 * Find `main()` function.
 
 
-### Spoiler
+### 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.
+
+
+### What this entry does:
 
 This program is a joke PDP-11 emulator.  ("ll-dpd" is embedded in the code.
 Read it upside down.)
@@ -80,7 +88,7 @@ You got it?  This program is supposed to be used in this way:
 You can enjoy the legendary winning entry of the first IOCCC, again.
 
 
-### Internal
+### How this entry works:
 
 This program uses `__attribute__((constructor))`, which is a GCC extension, to
 hijack the `main()` function call.  And then it interprets `main()` as an

2015/endoh3/index.html

@@ -438,7 +438,11 @@ the same reason. But I believe it is reasonably portable.</li>
 the source code from all angles..</li>
 <li>Find <code>main()</code> function.</li>
 </ul>
-<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="what-this-entry-does">What this entry does:</h3>
 <p>This program is a joke PDP-11 emulator. (“ll-dpd” is embedded in the code.
 Read it upside down.)</p>
 <p>You got it? This program is supposed to be used in this way:</p>
@@ -446,7 +450,7 @@ Read it upside down.)</p>
     clang -o prog prog.c mullender.c
     ./prog</code></pre>
 <p>You can enjoy the legendary winning entry of the first IOCCC, again.</p>
-<h3 id="internal">Internal</h3>
+<h3 id="how-this-entry-works">How this entry works:</h3>
 <p>This program uses <code>__attribute__((constructor))</code>, which is a GCC extension, to
 hijack the <code>main()</code> function call. And then it interprets <code>main()</code> as an
 instruction sequence.</p>