docs: add table of contents to README.md

Author: jessicayung

README.md

@@ -1,2 +1,26 @@
-# hackerrank
-Notes from HackerRank Challenges
+# programming-problems
+Notes and code from HackerRank Challenges and other programming problems.
+
+## Contents
+### Problem Areas
+Problems classified by area are from HackerRank. 
+
+**Finding these problems**: E.g. problems in the category 'Mathematics' are in a directory called 'mathematics'.
+* Algorithms
+* Functional Programming (Haskell)
+* Mathematics
+  * Primarily Project Euler problems
+* RegEx (Regular Expressions)
+* SQL
+* Statistics 
+  * HackerRank's Intro to Statistics Series
+
+### Problem Sources (non-HackerRank)
+These problems are not classified by area, but by source. These problems are not from HackerRank.
+* InterviewCake
+* Cracking the Coding Interview by Gabrielle Laakmann-McDowell (`laakmann-mcdowell`)
+
+### Flashcards
+Work-in-progress: flashcard content to import into Anki (flashcard software).
+* General programming terminology
+* Python syntax