include a gitignore file in exercises

[ashleygwilliams]

hey team!

i like keeping track of my work (even for exercises) on github. it'd be awesome if the exercises came with a .gitignore file (containing at least the target directory). this would make it very easy for students to store their work on github, and would also teach them what a good rust .gitignore looks like.

thanks for all you do! 🐶 🐬 ✨

[petertseng]

You know, this is when I think it would be great if we could have support for https://github.com/exercism/x-api/issues/29 so that we can have a language-wide gitignore that gets delivered to students at $EXERCISM_DIR/rust/.gitignore, instead of $EXERCISM_DIR/rust/$EXERCISE_SLUG/.gitignore

This is because I suspect that students are more likely to want to store all their Rust exercises in single repo, rather than one repository per exercise.