COMP: Add DoubleConversion merge from upstream documentation

DoubleConversion is a Google project, managed at this repository
https://code.google.com/p/double-conversion/

The txt files describe the procedure to merge upstream changes.
It's based on the work done for VNL; see
Modules/ThirdParty/VNL/src/README-ITK.txt

Change-Id: Ic1e958cfab2e7f1b057609a47946208540e6acd3
Merge topic 'AddDoubleConversionUpstreamMergeDocs'

d1e8446 COMP: Add DoubleConversion merge from upstream documentation