Suggest adding an antivirus folder exclusion on Windows

[jyn514]

I finally have a windows computer again and the first thing I immediately noticed is that even git status is ridiculously slow unless you disable antivirus scanning for the repo folder. We should suggest disabling scanning for the Rust repo only: https://support.microsoft.com/en-us/windows/add-an-exclusion-to-windows-security-811816c0-4dfd-af4a-47e4-c301afe13b26#ID0EBF=Windows_10