perf docs: Introduce security.txt file to document related issues

Publish instructions on how to apply LSM hooks for access control to
perf_event_open() syscall on Fedora distro with Targeted SELinux policy
and then manage access to the syscall.

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-security-module@vger.kernel.org
Cc: selinux@vger.kernel.org
Link: http://lore.kernel.org/lkml/290ded0a-c422-3749-5180-918fed1ee30f@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>