docs: process: Add cross-link to security-bugs

docs: process: Add cross-link to security-bugs

The submitting patches mentions criteria for a fix to be called
"security fix".  Add a link to document explaining the entire process
of handling security bugs.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Felipe Balbi <balbi@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200827105319.9734-1-krzk@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>