types-grammar, ch1: add text about boolean values

Author: getify

types-grammar/ch1.md

@@ -164,7 +164,29 @@ The `= ..` clause on a parameter is referred to as the "parameter default". It o
 
 The `boolean` type contains two values: `false` and `true`.
 
-// TODO
+In the "old days", programming languages would, by convention, use `0` to mean `false` and `1` to mean `true`. So you can think of the `boolean` type, and the keywords `false` and `true`, as a semantic convenience sugar on top of the `0` and `1` values:
+
+```js
+// isLoggedIn = 1;
+isLoggedIn = true;
+
+isComplete = 0;
+// isComplete = false;
+```
+
+Boolean values are how all decision making happens in a JS program:
+
+```js
+if (isLoggedIn) {
+    // do something
+}
+
+while (!isComplete) {
+    // keep going
+}
+```
+
+The `!` operator negates/flips a boolean value to the other one: `false` becomes `true`, and `true` becomes `false`.
 
 ### String Values