Update coding style doc

[ned21]

The coding style should be updated to reflect even more modern practices:

• use constant => Readonly
• Section "Be modular" talks about number of local variables, etc. Ideally it should be replaced with a mandate to make code unit testable since that tends to inherently force modularity. Readers interested in measures of technical complexity should be referred to read "Code Complete 2" instead!
• Add a section on writing good commit logs which should document the intent (or "why") of the change