Add a package doc to (try to) describe the purpose of this package, and
to describe the purpose (and expectations) of aliases provided by the
package.
> Package log provides types and functions related to logging, passing
> loggers through a context, and attaching context to the logger.
>
> # Transitional types
>
> This package contains various types that are aliases for types in [logrus].
> These aliases are intended for transitioning away from hard-coding logrus
> as logging implementation. Consumers of this package are encouraged to use
> the type-aliases from this package instead of directly using their logrus
> equivalent.
>
> The intent is to replace these aliases with locally defined types and
> interfaces once all consumers are no longer directly importing logrus
> types.
>
> IMPORTANT: due to the transitional purpose of this package, it is not
> guaranteed for the full logrus API to be provided in the future. As
> outlined, these aliases are provided as a step to transition away from
> a specific implementation which, as a result, exposes the full logrus API.
> While no decisions have been made on the ultimate design and interface
> provided by this package, we do not expect carrying "less common" features.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
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