Added some initial information giving an overview of the flow of the code.

docs/source/coding.rst

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+.. _coding:
+
+*************
+Code Overview
+*************
+
+This section aims to provide information and advice for developers who want to
+get started using and modifying the gprMax code.
+
+The code has been written in Python (3.x) with performance-critical parts, i.e.
+the FDTD solver, written using Cython (for CPU) or the NVIDIA CUDA programming
+model (for GPU). Cython allows the CPU-based solver to be parallelised using
+OpenMP which enables it to run on multi-core CPUs. gprMax also features a
+Messaging Passing Interface (MPI) task farm, which can operate with CPU nodes or
+multiple GPUs.
+
+.. figure:: images/code_overview.png
+
+    Basic, high-level overview of the flow of control (operation) of the code.