Update nucleotide-count description

Points out that understanding DNA is not necessary to complete the
exercise, as well as making the input format more clear.

exercism/problem-specifications#1716

Thanks to SleeplessByte, rpottsoh, kotp, ErikSchierboom, etc :)

exercism/problem-specifications#1719

exercises/nucleotide-count/README.md

@@ -1,16 +1,24 @@
 # Nucleotide Count
 
-Given a single stranded DNA string, compute how many times each nucleotide occurs in the string.
+Each of us inherits from our biological parents a set of chemical instructions known as DNA that influence how our bodies are constructed. All known life depends on DNA!
 
-The genetic language of every living thing on the planet is DNA.
-DNA is a large molecule that is built from an extremely long sequence of individual elements called nucleotides.
-4 types exist in DNA and these differ only slightly and can be represented as the following symbols: 'A' for adenine, 'C' for cytosine, 'G' for guanine, and 'T' thymine.
+> Note: You do not need to understand anything about nucleotides or DNA to complete this exercise.
 
-Here is an analogy:
-- twigs are to birds nests as
-- nucleotides are to DNA as
-- legos are to lego houses as
-- words are to sentences as...
+DNA is a long chain of other chemicals and the most important are the four nucleotides, adenine, cytosine, guanine and thymine. A single DNA chain can contain billions of these four nucleotides and the order in which they occur is important!
+We call the order of these nucleotides in a bit of DNA a "DNA sequence".
+
+We represent a DNA sequence as an ordered collection of these four nucleotides and a common way to do that is with a string of characters such as "ATTACG" for a DNA sequence of 6 nucleotides.
+'A' for adenine, 'C' for cytosine, 'G' for guanine, and 'T' for thymine.
+
+Given a string representing a DNA sequence, count how many of each nucleotide is present.
+If the string contains characters that aren't A, C, G, or T then it is invalid and you should signal an error.
+
+For example:
+
+```
+"GATTACA" -> 'A': 3, 'C': 1, 'G': 1, 'T': 2
+"INVALID" -> error
+```
 
 ## Rust Installation