Documentation: Basic usage with examples, limiting to specific buckets etc

[jonaseck2]

Would be great with an example containing:

• setting up gcsproxy with a storage bucket example. are all buckets exposed?
• managing access, guessing we need to do this using service accounts and IAM?
• what url will be exposed from gcsproxy given a bucket and object example.
• Limitations or similar

It would save some investigation time when choosing this solution over something else.