update cloud_config with instructions on required changes and add network.md, correct links in quick_start
clean up formatting
attempting in document relative link
adding one more hint for fusion users
Update coreos_cloud_config.md

Correcting relative link
Update networking.md

adding @jeinwei's experiences
Merge pull request #1784 from JaysonRaymond/master

CoreOS Getting Started Guide: should specify required cloud-config changes #1770