Add signed URL setup instructions in README

[toadjaune]

Hi !

First of all, thanks for providing this project ! It proved very useful :)

I found however the documentation on the required setup for use with signed URLs insufficient, and had to dig quite a bit in the code to understand how to make it work.

Here's a minimal doc addition explaining how to do it.

You might want to change the structure, such as adding it out of the docker section instead, including it in code samples, etc ...

Related issues : #631 #537

Closes #631.

[toadjaune]

Also, I think this way of doing things uses https://github.com/fsouza/fake-gcs-server/blob/main/fakestorage/server.go#L266, I'm not sure how to trigger the next two lines, maybe there's a cleaner way to do it.

[fsouza]

Also, I think this way of doing things uses https://github.com/fsouza/fake-gcs-server/blob/main/fakestorage/server.go#L266, I'm not sure how to trigger the next two lines, maybe there's a cleaner way to do it.

The second line (bucketHost) is for when the name of the bucket is specified in the host instead of the path (for example https://bucket.storage.googleapis.com/file.txt vs https://storage.googleapis.com/bucket/file.txt).

The third line is a special case: Google allows us you to name a bucket with a domain and point a CNAME to that bucket (see their docs for more details), so in that case we simply use the bucketName as the host.

[fsouza]

Thank you very much for contributing!