update packagers.md and kernel config check

Signed-off-by: Jessica Frazelle <acidburn@docker.com>

contrib/check-config.sh

@@ -183,6 +183,9 @@ echo 'Optional Features:'
 {
 	check_flags USER_NS
 }
+{
+	check_flags SECCOMP
+}
 {
 	check_flags MEMCG_KMEM MEMCG_SWAP MEMCG_SWAP_ENABLED
 	if  is_set MEMCG_SWAP && ! is_set MEMCG_SWAP_ENABLED; then

project/PACKAGERS.md

@@ -304,6 +304,7 @@ by having support for them in the kernel or userspace. A few examples include:
   least the "auplink" utility from aufs-tools)
 * BTRFS graph driver (requires BTRFS support enabled in the kernel)
 * ZFS graph driver (requires userspace zfs-utils and a corresponding kernel module)
+* Libseccomp to allow running seccomp profiles with containers
 
 ## Daemon Init Script