Update CONTRIBUTING.md
Update CONTRIBUTING.md

Fixed broken links.
tetrodoxin/NLog,littlesmilelove/NLog,luigiberrettini/NLog,ie-zero/NLog,304NotModified/NLog,BrutalCode/NLog,sean-gilliam/NLog,nazim9214/NLog,hubo0831/NLog,luigiberrettini/NLog,nazim9214/NLog,hubo0831/NLog,UgurAldanmaz/NLog,snakefoot/NLog,UgurAldanmaz/NLog,ie-zero/NLog,BrutalCode/NLog,littlesmilelove/NLog,tetrodoxin/NLog,NLog/NLog