update docs README

Author: Colin Wahl

docs/README.md

@@ -1,3 +1,13 @@
 # GitHub Actions Toolkit Documentation
 
 This directory contains documentation for GitHub Actions Toolkit. If you are interested in contributing new documentation, please read the [contributor guidelines](../.github/CONTRIBUTING.md) and [What Nobody Tells You About Documentation](https://documentation.divio.com) for help getting started.
+
+
+## Table of Contents
+
+1. [What is a GitHub Action?]("./01-What-Is-An-Action.md")
+2. [Core]("./02-Core.md")
+3. [Exec]("./03-Exec.md")
+4. [IO]("./04-IO.md")
+5. [Tool Cache]("./05-Tool-Cache.md")
+6. [Cache]("./06-Cache.md")