Add Spanish to list of audio cue translations

Spanish audio cues were added in
b641f3beb2d45afead301af81daea88151e29ecc.
Add note to CONTRIBUTING.md about code style

Based on comments in https://github.com/jonasoreland/runnerup/issues/57.
Add link to Android Studio build documentation
Merge pull request #60 from mattbostock/add-contributing-2

Further work on CONTRIBUTING.md