add .gitignore
edit .gitignore
Merge branch 'master' of https://github.com/shunyuan/GenesisPlus-libretro.git

# By Shunyuan Chou
# Via Shunyuan Chou
* 'master' of https://github.com/shunyuan/GenesisPlus-libretro.git:
  Update README.md