Document more for open source users using google cloud

This is for users running fuzzbench on their own, not service users.

We need to document:
1. How to ssh into runners (and soon workers).
This needs to be documented because trial runners (and soon workers) don't have external IPs, so we should document that you need to set up an SSH hop.

2. Document preemptible experiments.
We need to document there limitations but also strongly recommend them since most users will probably love the ~70-80% cost savings.