Expand documentation

[KnutZuidema]

Provide documentation describing the general use of each package
Provide documentation examples for all exported methods

[KnutZuidema]

Providing examples for API methods requires an API key, and since that key should not be public, I can't add it as a plain string in the example and providing an example with usage of an environment variable does not seem useful to me, because it would mean, that those examples can not be used in the documentation. There will be integration tests in the future, decoupled from the documentation which may be used as examples for the usage of the API methods.
See #15