Add strong naming key to repo

[LeeCampbell]

It appears to be recommended practice to do so

• https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/strong-name-signing.md
• https://msdn.microsoft.com/en-us/library/wd40t7ad.aspx

Might also be relevant to investigate Open Source Signing when the lib adopts .NET Core

• https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/oss-signing.md

[LeeCampbell]

Closed by https://github.com/HdrHistogram/HdrHistogram.NET/pull/20