Add more detailed API information to API section in README, provide mock keyring and backends implementation

[anadon]

Currently, I am writing tests which need a mock keyring. Looking at this project, the README seems to be the primary or only documentation for using this module. I see a few areas which would improve my experience in using this module. These are expanding the API documentation to include type annotations and more detailed explanations and constraints for parameters, a reference to or provided mock for keyring using monkey patching, and a reference to or provided mock for a backend generator able to provide multiple mock backends.

This is with acknowledgement that pytest-keyring appears insufficient for my needs.

[anadon]

Reading through the implementation more, it may not be apparent why I need mocks for something so simple. This is because my configuration project performs some complex handling and monkey patching around the keyring module that is supposed to be entirely transparent to a user, but actually intercepts all calls to the keyring module in order enforce some behavior constraints around changing backends and multiple backends and to support module import order independence.