Reverse String: Add emoji bonus to description

I've added an emoji bonus to the description of reverse string.
This introduces Unicode. Knowing that there are different ways to encode
text, and that not all characters are the same size is important.

I've made it a bonus because it's not straight forward in every
language.

Author: ccouzens

exercises/reverse-string/description.md

@@ -3,3 +3,27 @@ Reverse a string
 For example:
 input: "cool"
 output: "looc"
+
+## For bonus points
+
+Did you get the tests passing and the code clean? If you want to, these
+are some additional things you could try:
+
+- ASCII, UTF-8 and UTF-16 are 3 common character encodings you are likely to
+encounter as a programmer. ASCII is a character encoding published in 1963
+designed for representing English language text. UTF-8 and UTF-16 are both
+Unicode character encodings. Unicode was first published in 1991 and can now
+represent English language text, foreign text, historic text and emojis.
+
+  UTF-8 and UTF-16 both represent English language characters as a single code
+unit. But many emoji characters require 2 or more code units.
+
+  Add a test for reversing a string of a couple emoji characters. You may find
+that your existing solution has been reversing bytes or code units instead of
+characters.
+
+  Here is an example emoji string: 😀☕
+  And its reverse: ☕😀
+
+  Note that some languages are not Unicode aware. It is not recommended to try
+this challenge if your language does not have good Unicode support.