R-lifecycle: Add new package for version 0.2.0.

Manage the life cycle of your exported functions with shared
conventions, documentation badges, and non-invasive deprecation
warnings. The 'lifecycle' package defines four development stages
(experimental, maturing, stable, and questioning) and three
deprecation stages (soft-deprecated, deprecated, and defunct). It
makes it easy to insert badges corresponding to these stages in your
documentation. Usage of deprecated functions are signalled with
increasing levels of non-invasive verbosity.

(brook)