Add a note on building unit test project to README
Add a note on building unit test project to README

Emphasize that C# 6.0 support is required to build Unit Test project.
Remove trailing spaces.

ie-zero/NLog,luigiberrettini/NLog,sean-gilliam/NLog,304NotModified/NLog,NLog/NLog,UgurAldanmaz/NLog,hubo0831/NLog,BrutalCode/NLog,UgurAldanmaz/NLog,ie-zero/NLog,nazim9214/NLog,snakefoot/NLog,hubo0831/NLog,BrutalCode/NLog,nazim9214/NLog,luigiberrettini/NLog