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/README.md

@@ -64,10 +64,16 @@ This code has been compiled and tested on:
 * UNIX V7 pcc (Johnson's C compiler) on a [simulated PDP-11/45](http://simh.trailing-edge.com/)
   (see the "**SPOILER** section below for further detail)
 
-<div style="margin-bottom:60em;margin-top:4em"><strong>Spoiler below; scroll down.</strong></div>
+
+### 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.
 
 
-### SPOILER:
+### How this entry works:
 
 This started life as a personal challenge: write the shortest hex dump utility
 in C.