Added "Sum" to the Table of Contents (danrevah#14)

A link to the "Sum" example was missing from the Table of Contents. I have added it in.
sarahsga · Jan 12, 2017 · 54261be · 54261be
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commit 54261be
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diff --git a/README.md b/README.md
@@ -55,6 +55,7 @@
  - [Math](#math)   
     - [min](#min)
     - [max](#max)
+    - [sum](#sum)
     - [percentage](#percentage)
     - [ceil](#ceil)
     - [floor](#floor)