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/poikola/README.md

@@ -65,7 +65,15 @@ So I had to sit down and solve those tricky questions with Nano and a C
 compiler.
 
 
-### Little spoilers:
+### 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.
+
+
+### Little hints:
 
 Basically, the program draws animated ASCII art of the Big Dipper using Annie
 Jump Cannon's spectral classification system of stars and I think the colors of
@@ -121,7 +129,7 @@ mostly is okay. YMMV, as they say.
 Please do not feed little babies chocolate.
 
 
-### Major spoilers:
+### Major hints:
 
 I incorporated the [Fletcher 16 checksum
 algorithm](https://en.wikipedia.org/wiki/Fletcher%27s_checksum) into the source

2018/poikola/index.html

@@ -429,7 +429,11 @@ file <em>poikola.pdf</em>.</p>
 this star and may I have some Easter eggs?”</p>
 <p>So I had to sit down and solve those tricky questions with Nano and a C
 compiler.</p>
-<h3 id="little-spoilers">Little spoilers:</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="little-hints">Little hints:</h3>
 <p>Basically, the program draws animated ASCII art of the Big Dipper using Annie
 Jump Cannon’s spectral classification system of stars and I think the colors of
 the output are as accurate as possible.</p>
@@ -467,7 +471,7 @@ show anything at all. In other cases it won’t show all the colours right but
 mostly is okay. YMMV, as they say.</p>
 <h3 id="warnings-and-restrictions-required-by-law">Warnings and restrictions required by law:</h3>
 <p>Please do not feed little babies chocolate.</p>
-<h3 id="major-spoilers">Major spoilers:</h3>
+<h3 id="major-hints">Major hints:</h3>
 <p>I incorporated the <a href="https://en.wikipedia.org/wiki/Fletcher%27s_checksum">Fletcher 16 checksum
 algorithm</a> into the source
 for security reasons; it might be challenging to make changes without breaking