github: provide link to CVE process submission

Since project is hosted on GitHub, it makes sense to first
attempt to file issues there. Even ones that should
be submitted as part of CVE process.

This will help guide submitters to proper channels.

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: I1be5f8fb009ce06c71520675055801c392f3acfe
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1602


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>