Add walking/running on rocks sound

[nucleartide]

Process

1. Find a sound effect from freesound.org.
2. Check the sound's license to ensure that I can use it.
3. Import sound into Sounds/ folder.
4. Update Sounds/Attribution.md doc.
5. Expose new reference in the AllTheSounds ScriptableObject, and hook up the AudioClip. Alternatively, if you want to play the sound as its own AudioSource, then create the AudioSource and hook up the AudioClip.
6. Add test methods to SoundManager (or a separate script) that play the sound. If the audio clip is split into many sections, identify sections within the audio clip to play with the AudioClipSection class, then use AudioSourceHelpers.PlaySoundInterval.
7. Expose game event for when you want the sound to be played. For example, the PlayerAnimationEvents script exposes player animation events to C#.
8. Make SoundManager (or your separate script) listen to the new game event, and call the corresponding play-sound method when the event occurs.
9. Update readme with pull request link.