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.

2020/README.md

@@ -27,7 +27,7 @@ and [Snake - 2020/ferguson1](ferguson1/index.html).
 An entry, [kurdyukov1](kurdyukov1/index.html), pays homage to the previous entry
 [2015/hou](../2015/hou/index.html).
 
-...We'll stop spouting spoilers now. Have fun exploring all the entries!
+...We'll stop spouting too many details now. Have fun exploring all the entries!
 
 
 ## Remarks on some of submissions that did not win