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.

1994/horton/README.md

@@ -68,7 +68,15 @@ numbers to get a pleasing result, or use it for your high school algebra
 class.
 
 
-### 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:
 
 As should be obvious from 20 feet away, the program is a cubic plotter.
 It plots against certain artists, splattering their cubes with graphs
@@ -85,6 +93,9 @@ See the [gtface](%%REPO_URL%%/1994/horton/gtface.c) program for another example.
     ./gtface < gtface.data
 ```
 
+
+### How this entry works:
+
 For more information, see my article
 
 [Using GraphTab; USENIX ;login: Sept/Oct 1992, pages 28-31](login_sept92-pp28-31.pdf).