Style Guide: favor use of diamond constructor invocation

[javier-godoy]

The Flowing Code Style Guide v1.0.0 relies on Google Java Style Guide, which doesn't cover generics.

Particularly, it doesn't mention that diamond constructor invocation style is preferred.

I.e.

Map<String,List<Integer>> map = new HashMap<>();

Instead of:

Map<String,List<Integer>> map = new HashMap<String,List<Integer>>();

[mlopezFC]

I prefer the first one (less code), we should document it somewhere.