gitignore: .coverage added
gitignore: .coverage added

`.coverage` was added to exclude directory created on `tox` runs to hold
reports about code coverage.

Signed-off-by: Gergő Nagy <d3dcad219f7887bf0aa35b3eb99ea75d409b64fe@gmail.com>

hkrist/ratatoskr,ngergo/ratatoskr