add .gitignore with target/* on cargo init

[lattice0]

I always do cargo init and then there are hundreads of files from target/ waiting to be included in the commit. I always have to add target/* to a .gitignore file. Would be nice to have this added by default on cargo init

[Eh2406]

If you are not in an existing git repo, cargo init does start a git repo and adds a .gitignore. If you are already in a git repo maybe change the root .gitignore to include something like */target/*