Improve documentation

[Bluebugs]

Additional documentation of the upload process, selfupdatectl command and what should be present on the server for update process to succeed is needed.

[joec18]

I noticed there is a typo in the README. The README uses the following command as an example for upload:

selfupdatectl s3uploads myprogram-windows-amd64 targetS3PAth

This command has a typo in s3uploads rather it should be s3upload.

Futhermore I was able to figure out an example command to use. I did:

$ selfupdatectl s3upload --aws-bucket="bucket_name" --aws-region="us-east-1" --aws-secret="aws_secret_key" ID="aws_access_key" ./myapp.exe ./S3targetpath