types-grammar, ch1: adding footnote for 'signed zero' wikipedia page

Author: getify

types-grammar/ch1.md

@@ -835,7 +835,7 @@ Number.isSafeInteger(2 ** 53 - 1);  // true
 
 ### Double Zeros
 
-It may surprise you to learn that JS has two zeros: `0`, and `-0` (negative zero). But what on earth is a "negative zero"? A mathematician would surely balk at such a notion.
+It may surprise you to learn that JS has two zeros: `0`, and `-0` (negative zero). But what on earth is a "negative zero"? [^SignedZero] A mathematician would surely balk at such a notion.
 
 This isn't just a funny JS quirk; it's mandated by the IEEE-754[^IEEE754] specification. All floating point numbers are signed, including zero. And though JS does kind of hide the existence of `-0`, it's entirely possible to produce it and to detect it:
 
@@ -1119,3 +1119,5 @@ Before we move on to discussing JS's built-in object value type, we want to take
 [^IEEE754]: "IEEE-754"; https://en.wikipedia.org/wiki/IEEE_754 ; Accessed July 2022
 
 [^NumberType]: "6.1.6.1 The Number Type", ECMAScript 2022 Language Specification; https://262.ecma-international.org/13.0/#sec-ecmascript-language-types-number-type ; Accessed August 2022
+
+[^SignedZero]: "Signed Zero", Wikipedia; https://en.wikipedia.org/wiki/Signed_zero ; Accessed August 2022