Docs: Expectations for bug reporters and maintainers

Outline how often it's polite to ping kernel maintainers about bugs, and
suggest that kernel maintainers should respond to bugs in 1 to 5
business days.

Emphasize that regressions, userspace breakage, and kernel crashes are
exceptions to that rule.  Suggest escalation to LKML and Linus if these
bugs are ignored during the merge window.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>