Improve gitignoreFilter for use with flakes

[roberth]

https://github.com/hercules-ci/gitignore.nix/blob/master/docs/gitignoreFilter.md

• Better example
  • Flake
• Promote in README
• Improve flake attributes / interface if needed
  • Convenience function for .flakeignore?
  • flakeModule?

[cor]

flakeModule would be sweet