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 <[email protected]>

REPORTING-BUGS

@@ -1,3 +1,25 @@
+Background
+==========
+
+The upstream Linux kernel maintainers only fix bugs for specific kernel
+versions.  Those versions include the current "release candidate" (or -rc)
+kernel, any "stable" kernel versions, and any "long term" kernels.
+
+Please see https://www.kernel.org/ for a list of supported kernels.  Any
+kernel marked with [EOL] is "end of life" and will not have any fixes
+backported to it.
+
+If you've found a bug on a kernel version isn't listed on kernel.org,
+contact your Linux distribution or embedded vendor for support.
+Alternatively, you can attempt to run one of the supported stable or -rc
+kernels, and see if you can reproduce the bug on that.  It's preferable
+to reproduce the bug on the latest -rc kernel.
+
+
+How to report Linux kernel bugs
+===============================
+
+
 Identify the problematic subsystem
 ----------------------------------