Add derivation of thunk

Thunks have always confused me, and this is solely due to the fact that I never fully understood the word. Just knowing the specialized sense of a word does not clear up confusion when you have no idea where the word came from.

I have always been asking "but *why* is it called thunk?" This has honestly blown me off of Redux entirely for two years. I finally took the effort to find out.

I'm proposing we add the derivation of the term and its source, which will help greatly in clearing up understanding for people.

README.md

@@ -97,6 +97,7 @@ let x = 1 + 2;
 let foo = () => 1 + 2;
 ```
 
+The term [originated](https://en.wikipedia.org/wiki/Thunk#cite_note-1) as a humorous past-tense version of "think". 
 
 ## Installation