Docs: Add info on supported kernels to REPORTING-BUGS.

One of the most common frustrations maintainers have with bug reporters
is the email that starts with "I have a two year old kernel from an
embedded vendor with some random drivers and fixes thrown in, and it's
crashing".

Be specific about what kernel versions the upstream maintainers will fix
bugs in, and direct bug reporters to their Linux distribution or
embedded vendor if the bug is in an unsupported kernel.

Suggest that bug reporters should reproduce their bugs on the latest -rc
kernel.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>