[autoscaler] Document gcp subnet config (#3783)
[autoscaler] Document gcp subnet config (#3783)

Adds info to the gcp example yaml on using shared subnets.

stephanie-wang/ray,stephanie-wang/ray,richardliaw/ray,atumanov/ray,ujvl/ray-ng,richardliaw/ray,stephanie-wang/ray,richardliaw/ray,ujvl/ray-ng,robertnishihara/ray,atumanov/ray,richardliaw/ray,robertnishihara/ray,pcmoritz/ray-1,stephanie-wang/ray,stephanie-wang/ray,ujvl/ray-ng,ray-project/ray,ray-project/ray,atumanov/ray,atumanov/ray,ray-project/ray,robertnishihara/ray,stephanie-wang/ray,stephanie-wang/ray,richardliaw/ray,ray-project/ray,pcmoritz/ray-1,richardliaw/ray,richardliaw/ray,atumanov/ray,ray-project/ray,pcmoritz/ray-1,robertnishihara/ray,ray-project/ray,robertnishihara/ray,robertnishihara/ray,ray-project/ray,pcmoritz/ray-1,atumanov/ray,stephanie-wang/ray,pcmoritz/ray-1,pcmoritz/ray-1,ujvl/ray-ng,robertnishihara/ray,ray-project/ray,stephanie-wang/ray,pcmoritz/ray-1,atumanov/ray,atumanov/ray,robertnishihara/ray,pcmoritz/ray-1,robertnishihara/ray,richardliaw/ray