Add .editorconfig file ...

The .editorconfig file provides an editor agnostic definition of a
project's file format requirements.
Details can be found at http://editorconfig.org/

This change should have little impact on users, but, it is hoped, will
help those who move across projects avoid making mistakes because of
foreign project editor configuration settings.

Change-Id: I8776526b5ab96b543d3d3e445c60e06b62049e68
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>