docs: maintainer: document expectations of small time maintainers

docs: maintainer: document expectations of small time maintainers

We appear to have a gap in our process docs. We go into detail
on how to contribute code to the kernel, and how to be a subsystem
maintainer. I can't find any docs directed towards the thousands
of small scale maintainers, like folks maintaining a single driver
or a single network protocol.

Document our expectations and best practices. I'm hoping this doc
will be particularly useful to set expectations with HW vendors.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Martin Habets <habetsm.xilinx@gmail.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230719183225.1827100-1-kuba@kernel.org