update readme to have brief functions intro

.learn

@@ -4,6 +4,7 @@ tags:
   - if
   - else
   - lab
+  - functions
 languages:
   - javascript
 resources:

README.md

@@ -49,14 +49,17 @@ If we run the tests with the `learn` command, we see that
 `scuberGreetingForFeet` in the `index.js` file.
 
 ...but wait! We haven't gone over writing functions yet!  We will tackle the
-details of function writing in an upcoming lab. For now, briefly, a function
-declaration is written like so:
+details of function writing in depth in an upcoming lab. For now, briefly, a
+function declaration is written like so:
 
 ```js
 function addFive(someNumber) {
   //everything I want my function to do I put inside these curly braces
-  //in this example, I want my function, addFive, to add 5 to any number I pass in (someNumber)
-  let result = someNumber + 5;
+  //in this example, lets say I want my function, addFive, to add 5 to any number I pass in (someNumber), but only IF the number is greater than zero
+  let result
+  if (someNumber > 0) {
+    result = someNumber + 5;
+  }
   //at the end, if I want my function to return something, I need to state it:
   return result
 }
@@ -68,6 +71,8 @@ addFive(10);
 //returns 15
 addFive(20);
 //returns 25
+addFive(-5);
+//returns undefined
 addFive(addFive(5));
 //returns 15!! In this case, the return value of addFive(5), 10, is passed in as the argument to the outer addFive, returning 15
 ```