Add changelog file
Add changelog file

This change follows the suggestions from http://keepachangelog.com/
which proposes that it is much easier to note relevant changes in
a changelog at implementation time than trying to distilling them
from the git log at release time.

conniey/icu-dotnet,ermshiperete/icu-dotnet,ermshiperete/icu-dotnet,conniey/icu-dotnet,sillsdev/icu-dotnet,sillsdev/icu-dotnet