add .gitattributes for GitHub Linguist (#87)

Some .v files are incorrectly identified by GitHub Linguist as Coq or Vlang
files, which confuses people who browse the repo on GitHub. According to
https://github.com/github/linguist#using-gitattributes adding a .gitattributes
file can solve this problem.

.gitattributes

+*.v linguist-language=Verilog