HACKING.md: Add documentation for ESLint
HACKING.md: Add documentation for ESLint

Closes #9038
deryni/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,deryni/cockpit,cockpit-project/cockpit,cockpituous/cockpit,cockpituous/cockpit,deryni/cockpit,stefwalter/cockpit,andreasn/cockpit,garrett/cockpit,martinpitt/cockpit,stefwalter/cockpit,moraleslazaro/cockpit,deryni/cockpit,SotolitoLabs/cockpit,SotolitoLabs/cockpit,garrett/cockpit,stefwalter/cockpit,cockpituous/cockpit,stefwalter/cockpit,garrett/cockpit,andreasn/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,mvollmer/cockpit,mareklibra/cockpit,mareklibra/cockpit,mareklibra/cockpit,martinpitt/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,SotolitoLabs/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,martinpitt/cockpit,cockpit-project/cockpit,andreasn/cockpit,stefwalter/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,garrett/cockpit,deryni/cockpit,moraleslazaro/cockpit,garrett/cockpit,stefwalter/cockpit,andreasn/cockpit,mareklibra/cockpit,mvollmer/cockpit,cockpituous/cockpit,cockpit-project/cockpit,andreasn/cockpit,stefwalter/cockpit,mareklibra/cockpit,andreasn/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,deryni/cockpit,mareklibra/cockpit,cockpituous/cockpit,mareklibra/cockpit,deryni/cockpit,andreasn/cockpit,moraleslazaro/cockpit