upd readmes, rename exercises.

chapter3/README.md

@@ -0,0 +1,10 @@
+# Recursion
+
+Recursive functions must have both:
+
+-   one or more base cases
+-   a recursive case
+
+The base cases are required to ensure the recursion stops when meeting a condition
+
+The recursive case adds functions onto the call stack and completes each one top down.

chapter4/README.md

@@ -1,12 +1,9 @@
-# Recursion
+# Quicksort
 
-Recursive functions must have both:
+Similar to the previous recursive function, quicksort uses divide and conquer.
 
--   one or more base cases
--   a recursive case
+The base case occurs for an array size 0 or 1 (doesn't need to be sorted).
 
-The base cases are required to ensure the recursion stops when meeting a condition
-
-The recursive case adds functions onto the call stack and completes each one top down.
+The recursive case works by partitioning the array around a chosen pivot repeatedly until the base case is met and then combining all sorted sub-arrays.
 
 Note. Quicksort should be implemented using a random pivot to ensure average runtimes.