Added introduction.md for loops.

concepts/loops/about.md

@@ -12,9 +12,6 @@ The keywords `break`, `continue`, and `else` help customize loop behavior.
 
 ```python
 
-# Lists are considered "truthy" in a boolean context if they
-# contain one or more values, and "falsy" if they are empty.
-
 >>> placeholders = ["spam", "ham", "eggs", "green_spam", "green_ham", "green_eggs"]
 
 >>> while 'eggs' in placeholders:

concepts/loops/introduction.md

@@ -3,6 +3,8 @@
 There are two looping constructs.
 `while` loops for _indefinite_ (uncounted) iteration and `for` loops for  _definite_, (counted) iteration.
 The keywords `break`, `continue`, and `else` help customize loop behavior.
+`range()` and `enumerate()` help with loop counting and indexing.
+
 
 `while` loops continue to exectute as long as the loop expression or "test" evaluates to `True`, terminating when it evaluates to `False`.
 
@@ -17,3 +19,5 @@ while expression:
 for item in sequence:
     set_of_statements
 ```
+
+