Added the Language Agnostic subsections to TOC

Found myself scrolling through the Language Agnostic section a lot; added TOC for quicker access.

free-programming-books.md

@@ -3,6 +3,23 @@
 * [Graphics user interfaces](#graphics-user-interfaces)
 * [Graphics Programming](#graphics-programming)
 * [Language Agnostic](#language-agnostic)
+    * [Algorithms & Datastructures](#algorithms--datastructures)
+    * [Theoretical Computer Science](#theoretical-computer-science)
+    * [Operating systems](#operating-systems)
+    * [Database](#database)
+    * [Networking](#networking)
+    * [Compiler Design](#compiler-design)
+    * [Programming Paradigms](#programming-paradigms)
+    * [Parallel Programming](#parallel-programming)
+    * [Software Architecture](#software-architecture)
+    * [Open Source Ecosystem](#open-source-ecosystem)
+    * [Information Retrieval](#information-retrieval)
+    * [Datamining](#datamining)
+    * [Machine Learning](#machine-learning)
+    * [Mathematics](#mathematics)
+    * [Misc](#misc)
+    * [Web Performance](#web-performance)
+    * [MOOC](#mooc)
 * [Ada](#ada)
 * [Android](#android)
 * [Autotools](#autotools)