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.

2000/primenum/README.md

@@ -34,7 +34,6 @@ NOTE: `n` is an integer.
 
 ## Judges' remarks:
 
-Don't read the author's remarks until you're ready for the spoiler.
 Just stop and read the program; it won't take long.  Once you've figured
 out what it does, run it.  It may surprise you.
 
@@ -42,6 +41,14 @@ This entry won, not because the obfuscation is impenetrable, but because
 it successfully fooled half of the judges, including me.
 
 
+### 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.
+
+
 ## Author's remarks:
 
 ### Description