Document credential refreshing behaviour

[ShaddyDC]

Is your feature request related to a problem? Please describe.
When using this library and playing around with the credentials, I noticed the expiration field on the Credentials struct. Together with the public refresh function and no further documentation, I figured that was something I'd have to take care of myself. Only when checking the issues here again just to be sure did I find #163 and saw that it's being done automatically.

Describe the solution you'd like
Just a simple note on Bucket::credentials_refresh, Credentials::refresh or Credentials::expiration that the refresh is being done automatically and needs not be considered by the user excepting special circumstances.

Describe alternatives you've considered

• Mention automatic credential renewals as a feature in the readme
• Make the refresh methods and expiration private