add Py_UNREACHABLE() CPython macro Easter egg

Add "Unreachable State" entry documenting some funny error messages
in CPython's Py_UNREACHABLE() macro, including the `RANDALL_WAS_HERE`
build flag referencing xkcd author Randall Munroe.

Author: j4ckofalltrades

README.md

@@ -213,6 +213,32 @@ Hello World!
 >>> import __phello__.ham.eggs
 ```
 
+### 17. Unreachable State[^unreachable-state]
+This is a macro in CPython (Python's reference implementation) source code used to mark code paths that should never execute.
+
+```c
+#if defined(RANDALL_WAS_HERE)
+#  define Py_UNREACHABLE() \
+    Py_FatalError( \
+        "If you're seeing this, the code is in what I thought was\n" \
+        "an unreachable state.\n\n" \
+        "I could give you advice for what to do, but honestly, why\n" \
+        "should you trust me?  I clearly screwed this up.  I'm writing\n" \
+        "a message that should never appear, yet I know it will\n" \
+        "probably appear someday.\n\n" \
+        "On a deep level, I know I'm not up to this task.\n" \
+        "I'm so sorry.\n" \
+        "https://xkcd.com/2200")
+#elif defined(Py_DEBUG)
+#  define Py_UNREACHABLE() \
+    Py_FatalError( \
+        "We've reached an unreachable state. Anything is possible.\n" \
+        "The limits were in our heads all along. Follow your dreams.\n" \
+        "https://xkcd.com/2200")
+// ...
+#endif
+```
+
 [^hello-world]: Easiest hello world program in a language without calling any function
 [^the-classic]: Each and every line is the philosophy of Python's design and is a supreme holy guide
 [^missing-zen]: Maybe just to show that there always should be a new line at the end of a file!
@@ -228,6 +254,7 @@ Hello World!
 [^inpynite]: [Source](https://www.reddit.com/r/Python/comments/6wrd8t/nice_lil_easter_egg_i_suppose/).
 [^peg-parser]: See [this answer](https://stackoverflow.com/a/65487013/14362510)
 [^breakfast-of-champions]: [Frozen modules](https://github.com/python/cpython/blob/main/Python/frozen.c) used for testing and is a reference to Monty Python's [SPAM sketch](https://cs.stanford.edu/people/eroberts/cs181/projects/spam/sketch.HTML)
+[^unreachable-state]: `Py_UNREACHABLE()` is a macro in CPython's [Include/pymacro.h](https://github.com/python/cpython/blob/main/Include/pymacro.h#L243-L270) used to mark code paths that should never execute. The `RANDALL_WAS_HERE` build flag refers to Randall Munroe, the xkcd webcomic author. The error messages that should never be printed are a reference to this [xkcd comic](https://xkcd.com/2200).
 
 ## Add more